## Date and Time Functions

Date and Time Functions — calendrical calculations and miscellaneous time stuff.

## Synopsis

    #include <glib.h>

    #define     G_USEC_PER_SEC
    struct      GTimeVal;
    void        g_get_current_time              (GTimeVal *result);
    void        g_usleep                        (gulong microseconds);
    void        g_time_val_add                  (GTimeVal *time_,
                                                 glong microseconds);

    struct      GDate;
    typedef     GTime;
    enum        GDateDMY;
    typedef     GDateDay;
    enum        GDateMonth;
    typedef     GDateYear;
    enum        GDateWeekday;

    #define     G_DATE_BAD_DAY
    #define     G_DATE_BAD_JULIAN
    #define     G_DATE_BAD_YEAR

    GDate*      g_date_new                      (void);
    GDate*      g_date_new_dmy                  (GDateDay day,
                                                 GDateMonth month,
                                                 GDateYear year);
    GDate*      g_date_new_julian               (guint32 julian_day);
    void        g_date_clear                    (GDate *date,
                                                 guint n_dates);
    void        g_date_free                     (GDate *date);

    void        g_date_set_day                  (GDate *date,
                                                 GDateDay day);
    void        g_date_set_month                (GDate *date,
                                                 GDateMonth month);
    void        g_date_set_year                 (GDate *date,
                                                 GDateYear year);
    void        g_date_set_dmy                  (GDate *date,
                                                 GDateDay day,
                                                 GDateMonth month,
                                                 GDateYear y);
    void        g_date_set_julian               (GDate *date,
                                                 guint32 julian_date);
    void        g_date_set_time                 (GDate *date,
                                                 GTime time_);
    void        g_date_set_parse                (GDate *date,
                                                 const gchar *str);

    void        g_date_add_days                 (GDate *date,
                                                 guint n_days);
    void        g_date_subtract_days            (GDate *date,
                                                 guint n_days);
    void        g_date_add_months               (GDate *date,
                                                 guint n_months);
    void        g_date_subtract_months          (GDate *date,
                                                 guint n_months);
    void        g_date_add_years                (GDate *date,
                                                 guint n_years);
    void        g_date_subtract_years           (GDate *date,
                                                 guint n_years);
    gint        g_date_days_between             (const GDate *date1,
                                                 const GDate *date2);
    gint        g_date_compare                  (const GDate *lhs,
                                                 const GDate *rhs);
    void        g_date_clamp                    (GDate *date,
                                                 const GDate *min_date,
                                                 const GDate *max_date);
    void        g_date_order                    (GDate *date1,
                                                 GDate *date2);

    GDateDay    g_date_get_day                  (const GDate *date);
    GDateMonth  g_date_get_month                (const GDate *date);
    GDateYear   g_date_get_year                 (const GDate *date);
    guint32     g_date_get_julian               (const GDate *date);
    GDateWeekday g_date_get_weekday             (const GDate *date);
    guint       g_date_get_day_of_year          (const GDate *date);

    guint8      g_date_get_days_in_month        (GDateMonth month,
                                                 GDateYear year);
    gboolean    g_date_is_first_of_month        (const GDate *date);
    gboolean    g_date_is_last_of_month         (const GDate *date);
    gboolean    g_date_is_leap_year             (GDateYear year);
    guint       g_date_get_monday_week_of_year  (const GDate *date);
    guint8      g_date_get_monday_weeks_in_year (GDateYear year);
    guint       g_date_get_sunday_week_of_year  (const GDate *date);
    guint8      g_date_get_sunday_weeks_in_year (GDateYear year);

    gsize       g_date_strftime                 (gchar *s,
                                                 gsize slen,
                                                 const gchar *format,
                                                 const GDate *date);
    void        g_date_to_struct_tm             (const GDate *date,
                                                 struct tm *tm);

    gboolean    g_date_valid                    (const GDate *date);
    gboolean    g_date_valid_day                (GDateDay day);
    gboolean    g_date_valid_month              (GDateMonth month);
    gboolean    g_date_valid_year               (GDateYear year);
    gboolean    g_date_valid_dmy                (GDateDay day,
                                                 GDateMonth month,
                                                 GDateYear year);
    gboolean    g_date_valid_julian             (guint32 julian_date);
    gboolean    g_date_valid_weekday            (GDateWeekday weekday);

## Description

The GDate data structure represents a day between January 1, Year 1, and sometime a few thousand years in the future (right now it will go to the year 65535 or so, but `g_date_set_parse()` only parses up to the year 8000 or so - just count on "a few thousand"). GDate is meant to represent everyday dates, not astronomical dates or historical dates or ISO timestamps or the like. It extrapolates the current Gregorian calendar forward and backward in time; there is no attempt to change the calendar to match time periods or locations. GDate does not store time information; it represents a day.

The GDate implementation has several nice features; it is only a 64-bit struct, so storing large numbers of dates is very efficient. It can keep both a Julian and day-month-year representation of the date, since some calculations are much easier with one representation or the other. A Julian representation is simply a count of days since some fixed day in the past; for GDate the fixed day is January 1, 1 AD. ("Julian" dates in the GDate API aren't really Julian dates in the technical sense; technically, Julian dates count from the start of the Julian period, Jan 1, 4713 BC).

GDate is simple to use. First you need a "blank" date; you can get a dynamically allocated date from `g_date_new()`, or you can declare an automatic variable or array and initialize it to a sane state by calling `g_date_clear()`. A cleared date is sane; it's safe to call `g_date_set_dmy()` and the other mutator functions to initialize the value of a cleared date. However, a cleared date is initially invalid, meaning that it doesn't represent a day that exists. It is undefined to call any of the date calculation routines on an invalid date. If you obtain a date from a user or other unpredictable source, you should check its validity with the `g_date_valid()` predicate. `g_date_valid()` is also used to check for errors with `g_date_set_parse()` and other functions that can fail. Dates can be invalidated by calling `g_date_clear()` again.

It is very important to use the API to access the GDate struct. Often only the day-month-year or only the Julian representation is valid. Sometimes neither is valid. Use the API.

GLib doesn't contain any time-manipulation functions; however, there is a GTime typedef which is equivalent to time_t, and a GTimeVal struct which represents a more precise time (with microseconds). You can request the current time as a GTimeVal with `g_get_current_time()`.

## Details

### G_USEC_PER_SEC

    #define G_USEC_PER_SEC 1000000

Number of microseconds in one second (1 million). This macro is provided for code readability.

### struct GTimeVal

    struct GTimeVal {

      glong tv_sec;
      glong tv_usec;
    };

Represents a precise time, with seconds and microseconds. Same as the struct timeval returned by the ``gettimeofday()`` UNIX call.

glong `tv_sec` | seconds.
glong `tv_usec` | microseconds.

### g_get_current_time ()

    void        g_get_current_time              (GTimeVal *result);

Equivalent to the UNIX `gettimeofday()` function, but portable.

`result` : | GTimeVal structure in which to store current time.

### g_usleep ()

    void        g_usleep                        (gulong microseconds);

Pauses the current thread for the given number of microseconds. There are 1 million microseconds per second (represented by the G_USEC_PER_SEC macro). `g_usleep()` may have limited precision, depending on hardware and operating system; don't rely on the exact length of the sleep.

`microseconds` : | number of microseconds to pause.

### g_time_val_add ()

    void        g_time_val_add                  (GTimeVal *time_,
                                                 glong microseconds);

Adds the given number of microseconds to `time_`. `microseconds` can also be negative to decrease the value of `time_`.

`time_` : | a GTimeVal
`microseconds` : | number of microseconds to add to `time`

### struct GDate

    struct GDate {

      guint julian_days : 32; /* julian days representation - we use a
                               *  bitfield hoping that 64 bit platforms
                               *  will pack this whole struct in one big
                               *  int
                               */

      guint julian : 1;    /* julian is valid */
      guint dmy    : 1;    /* dmy is valid */

      /* DMY representation */
      guint day    : 6;
      guint month  : 4;
      guint year   : 16;
    };

Represents a day between January 1, Year 1 and a few thousand years in the future. None of its members should be accessed directly. If the GDate is obtained from `g_date_new()`, it will be safe to mutate but invalid and thus not safe for calendrical computations. If it's declared on the stack, it will contain garbage so must be initialized with `g_date_clear()`. `g_date_clear()` makes the date invalid but sane. An invalid date doesn't represent a day, it's "empty." A date becomes valid after you set it to a Julian day or you set a day, month, and year.

### GTime

    typedef gint32  GTime;

Simply a replacement for time_t. Unrelated to GTimer.

### enum GDateDMY

    typedef enum
    {
      G_DATE_DAY   = 0,
      G_DATE_MONTH = 1,
      G_DATE_YEAR  = 2
    } GDateDMY;

This enumeration isn't used in the API, but may be useful if you need to mark a number as a day, month, or year.

### GDateDay

    typedef guint8  GDateDay;   /* day of the month */

Integer representing a day of the month; between 1 and 31. G_DATE_BAD_DAY represents an invalid day of the month.

### enum GDateMonth

    typedef enum
    {
      G_DATE_BAD_MONTH = 0,
      G_DATE_JANUARY   = 1,
      G_DATE_FEBRUARY  = 2,
      G_DATE_MARCH     = 3,
      G_DATE_APRIL     = 4,
      G_DATE_MAY       = 5,
      G_DATE_JUNE      = 6,
      G_DATE_JULY      = 7,
      G_DATE_AUGUST    = 8,
      G_DATE_SEPTEMBER = 9,
      G_DATE_OCTOBER   = 10,
      G_DATE_NOVEMBER  = 11,
      G_DATE_DECEMBER  = 12
    } GDateMonth;

Enumeration representing a month; values are G_DATE_JANUARY, G_DATE_FEBRUARY, etc. G_DATE_BAD_MONTH is the invalid value.

`G_DATE_BAD_MONTH` | invalid value.
`G_DATE_JANUARY` | January.
`G_DATE_FEBRUARY` | February.
`G_DATE_MARCH` | March.
`G_DATE_APRIL` | April.
`G_DATE_MAY` | May.
`G_DATE_JUNE` | June.
`G_DATE_JULY` | July.
`G_DATE_AUGUST` | August.
`G_DATE_SEPTEMBER` | September.
`G_DATE_OCTOBER` | October.
`G_DATE_NOVEMBER` | November.
`G_DATE_DECEMBER` | December.

### GDateYear

    typedef guint16 GDateYear;

Integer representing a year; G_DATE_BAD_YEAR is the invalid value. The year must be 1 or higher; negative (BC) years are not allowed. The year is represented with four digits.

### enum GDateWeekday

    typedef enum
    {
      G_DATE_BAD_WEEKDAY  = 0,
      G_DATE_MONDAY       = 1,
      G_DATE_TUESDAY      = 2,
      G_DATE_WEDNESDAY    = 3,
      G_DATE_THURSDAY     = 4,
      G_DATE_FRIDAY       = 5,
      G_DATE_SATURDAY     = 6,
      G_DATE_SUNDAY       = 7
    } GDateWeekday;

Enumeration representing a day of the week; G_DATE_MONDAY, G_DATE_TUESDAY, etc. G_DATE_BAD_WEEKDAY is an invalid weekday.

`G_DATE_BAD_WEEKDAY` | invalid value.
`G_DATE_MONDAY` | Monday.
`G_DATE_TUESDAY` | Tuesday.
`G_DATE_WEDNESDAY` | Wednesday.
`G_DATE_THURSDAY` | Thursday.
`G_DATE_FRIDAY` | Friday.
`G_DATE_SATURDAY` | Saturday.
`G_DATE_SUNDAY` | Sunday.

### G_DATE_BAD_DAY

    #define G_DATE_BAD_DAY    0U

Represents an invalid GDateDay.

### G_DATE_BAD_JULIAN

    #define G_DATE_BAD_JULIAN 0U

Represents an invalid Julian day number.

### G_DATE_BAD_YEAR

    #define G_DATE_BAD_YEAR   0U

Represents an invalid year.

### g_date_new ()

    GDate*      g_date_new                      (void);

Allocates a GDate and initializes it to a sane state. The new date will be cleared (as if you'd called `g_date_clear()`) but invalid (it won't represent an existing day). Free the return value with `g_date_free()`.

Returns : | a newly-allocated GDate.

### g_date_new_dmy ()

    GDate*      g_date_new_dmy                  (GDateDay day,
                                                 GDateMonth month,
                                                 GDateYear year);

Like `g_date_new()`, but also sets the value of the date. Assuming the day-month-year triplet you pass in represents an existing day, the returned date will be valid.

`day` : | day of the month.
`month` : | month of the year.
`year` : | year
Returns : | a newly-allocated GDate initialized with `day`, `month`, and `year`.

### g_date_new_julian ()

    GDate*      g_date_new_julian               (guint32 julian_day);

Like `g_date_new()`, but also sets the value of the date. Assuming the Julian day number you pass in is valid (greater than 0, less than an unreasonably large number), the returned date will be valid.

`julian_day` : | days since January 1, Year 1.
Returns : | a newly-allocated GDate initialized with `julian_day`.

### g_date_clear ()

    void        g_date_clear                    (GDate *date,
                                                 guint n_dates);

Initializes one or more GDate structs to a sane but invalid state. The cleared dates will not represent an existing date, but will not contain garbage. Useful to init a date declared on the stack. Validity can be tested with `g_date_valid()`.

`date` : | pointer to one or more dates to clear.
`n_dates` : | number of dates to clear.

### g_date_free ()

    void        g_date_free                     (GDate *date);

Frees a GDate returned from `g_date_new()`.

`date` : | a GDate.

### g_date_set_day ()

    void        g_date_set_day                  (GDate *date,
                                                 GDateDay day);

Sets the day of the month for a GDate. If the resulting day-month-year triplet is invalid, the date will be invalid.

`date` : | a GDate.
`day` : | day to set.

### g_date_set_month ()

    void        g_date_set_month                (GDate *date,
                                                 GDateMonth month);

Sets the month of the year for a GDate. If the resulting day-month-year triplet is invalid, the date will be invalid.

`date` : | a GDate.
`month` : | month to set.

### g_date_set_year ()

    void        g_date_set_year                 (GDate *date,
                                                 GDateYear year);

Sets the year for a GDate. If the resulting day-month-year triplet is invalid, the date will be invalid.

`date` : | a GDate.
`year` : | year to set.

### g_date_set_dmy ()

    void        g_date_set_dmy                  (GDate *date,
                                                 GDateDay day,
                                                 GDateMonth month,
                                                 GDateYear y);

Sets the value of a GDate from a day, month, and year. The day-month-year triplet must be valid; if you aren't sure it is, call `g_date_valid_dmy()` to check before you set it.

`date` : | a GDate.
`day` : | day.
`month` : | month.
`y` : | year.

### g_date_set_julian ()

    void        g_date_set_julian               (GDate *date,
                                                 guint32 julian_date);

Sets the value of a GDate from a Julian day number.

`date` : | a GDate.
`julian_date` : | Julian day number (days since January 1, Year 1).

### g_date_set_time ()

    void        g_date_set_time                 (GDate *date,
                                                 GTime time_);

Sets the value of a date from a GTime (time_t) value. To set the value of a date to the current day, you could write:

    g_date_set_time (date, time (NULL));

`date` : | a GDate.
`time_` : | GTime value to set.

### g_date_set_parse ()

    void        g_date_set_parse                (GDate *date,
                                                 const gchar *str);

Parses a user-inputted string `str`, and try to figure out what date it represents, taking the current locale into account. If the string is successfully parsed, the date will be valid after the call. Otherwise, it will be invalid. You should check using `g_date_valid()` to see whether the parsing succeeded.

This function is not appropriate for file formats and the like; it isn't very precise, and its exact behavior varies with the locale. It's intended to be a heuristic routine that guesses what the user means by a given string (and it does work pretty well in that capacity).

`date` : | a GDate to fill in.
`str` : | string to parse.

### g_date_add_days ()

    void        g_date_add_days                 (GDate *date,
                                                 guint n_days);

Increments a date some number of days. To move forward by weeks, add weeks*7 days. The date must be valid.

`date` : | a GDate to increment.
`n_days` : | number of days to move the date forward.

### g_date_subtract_days ()

    void        g_date_subtract_days            (GDate *date,
                                                 guint n_days);

Moves a date some number of days into the past. To move by weeks, just move by weeks*7 days. The date must be valid.

`date` : | a GDate to decrement.
`n_days` : | number of days to move.

### g_date_add_months ()

    void        g_date_add_months               (GDate *date,
                                                 guint n_months);

Increments a date by some number of months. If the day of the month is greater than 28, this routine may change the day of the month (because the destination month may not have the current day in it). The date must be valid.

`date` : | a GDate to increment.
`n_months` : | number of months to move forward.

### g_date_subtract_months ()

    void        g_date_subtract_months          (GDate *date,
                                                 guint n_months);

Moves a date some number of months into the past. If the current day of the month doesn't exist in the destination month, the day of the month may change. The date must be valid.

`date` : | a GDate to decrement.
`n_months` : | number of months to move.

### g_date_add_years ()

    void        g_date_add_years                (GDate *date,
                                                 guint n_years);

Increments a date by some number of years. If the date is February 29, and the destination year is not a leap year, the date will be changed to February 28. The date must be valid.

`date` : | a GDate to increment.
`n_years` : | number of years to move forward.

### g_date_subtract_years ()

    void        g_date_subtract_years           (GDate *date,
                                                 guint n_years);

Moves a date some number of years into the past. If the current day doesn't exist in the destination year (i.e. it's February 29 and you move to a non-leap-year) then the day is changed to February 29. The date must be valid.

`date` : | a GDate to decrement.
`n_years` : | number of years to move.

### g_date_days_between ()

    gint        g_date_days_between             (const GDate *date1,
                                                 const GDate *date2);

Computes the number of days between two dates. If `date2` is prior to `date1`, the returned value is negative. Both dates must be valid.

`date1` : | the first date.
`date2` : | the second date.
Returns : | the number of days between `date1` and `date2`.

### g_date_compare ()

    gint        g_date_compare                  (const GDate *lhs,
                                                 const GDate *rhs);

``qsort()``-style comparsion function for dates. Both dates must be valid.

`lhs` : | first date to compare.
`rhs` : | second date to compare.
Returns : | 0 for equal, less than zero if `lhs` is less than `rhs`, greater than zero if `lhs` is greater than `rhs`.

### g_date_clamp ()

    void        g_date_clamp                    (GDate *date,
                                                 const GDate *min_date,
                                                 const GDate *max_date);

If `date` is prior to `min_date`, sets `date` equal to `min_date`. If `date` falls after `max_date`, sets `date` equal to `max_date`. Either `min_date` and `max_date` may be `NULL`. All non-`NULL` dates must be valid.

`date` : | a GDate to clamp.
`min_date` : | minimum accepted value for `date`.
`max_date` : | maximum accepted value for `date`.

### g_date_order ()

    void        g_date_order                    (GDate *date1,
                                                 GDate *date2);

Checks if `date1` is less than or equal to `date2`, and swap the values if this is not the case.

`date1` : | the first date.
`date2` : | the second date.

### g_date_get_day ()

    GDateDay    g_date_get_day                  (const GDate *date);

Returns the day of the month. The date must be valid.

`date` : | a GDate to extract the day of the month from.
Returns : | day of the month.

### g_date_get_month ()

    GDateMonth  g_date_get_month                (const GDate *date);

Returns the month of the year. The date must be valid.

`date` : | a GDate to get the month from.
Returns : | month of the year as a GDateMonth.

### g_date_get_year ()

    GDateYear   g_date_get_year                 (const GDate *date);

Returns the year of a GDate. The date must be valid.

`date` : | a GDate.
Returns : | year in which the date falls.

### g_date_get_julian ()

    guint32     g_date_get_julian               (const GDate *date);

Returns the Julian day or "serial number" of the GDate. The Julian day is simply the number of days since January 1, Year 1; i.e., January 1, Year 1 is Julian day 1; January 2, Year 1 is Julian day 2, etc. The date must be valid.

`date` : | a GDate to extract the Julian day from.
Returns : | Julian day.

### g_date_get_weekday ()

    GDateWeekday g_date_get_weekday             (const GDate *date);

Returns the day of the week for a GDate. The date must be valid.

`date` : | a GDate.
Returns : | day of the week as a GDateWeekday.

### g_date_get_day_of_year ()

    guint       g_date_get_day_of_year          (const GDate *date);

Returns the day of the year, where Jan 1 is the first day of the year. The date must be valid.

`date` : | a GDate to extract day of year from.
Returns : | day of the year.

### g_date_get_days_in_month ()

    guint8      g_date_get_days_in_month        (GDateMonth month,
                                                 GDateYear year);

Returns the number of days in a month, taking leap years into account.

`month` : | month.
`year` : | year.
Returns : | number of days in `month` during the `year`.

### g_date_is_first_of_month ()

    gboolean    g_date_is_first_of_month        (const GDate *date);

Returns `TRUE` if the date is on the first of a month. The date must be valid.

`date` : | a GDate to check.
Returns : | `TRUE` if the date is the first of the month.

### g_date_is_last_of_month ()

    gboolean    g_date_is_last_of_month         (const GDate *date);

Returns `TRUE` if the date is the last day of the month. The date must be valid.

`date` : | a GDate to check.
Returns : | `TRUE` if the date is the last day of the month.

### g_date_is_leap_year ()

    gboolean    g_date_is_leap_year             (GDateYear year);

Returns `TRUE` if the year is a leap year.

`year` : | year to check.
Returns : | `TRUE` if the year is a leap year.

### g_date_get_monday_week_of_year ()

    guint       g_date_get_monday_week_of_year  (const GDate *date);

Returns the week of the year, where weeks are understood to start on Monday. If the date is before the first Monday of the year, return 0. The date must be valid.

`date` : | a GDate.
Returns : | week of the year.

### g_date_get_monday_weeks_in_year ()

    guint8      g_date_get_monday_weeks_in_year (GDateYear year);

Returns the number of weeks in the year, where weeks are taken to start on Monday. Will be 52 or 53. The date must be valid. (Years always have 52 7-day periods, plus 1 or 2 extra days depending on whether it's a leap year. This function is basically telling you how many Mondays are in the year, i.e. there are 53 Mondays if one of the extra days happens to be a Monday.)

`year` : | a year.
Returns : | number of Mondays in the year.

### g_date_get_sunday_week_of_year ()

    guint       g_date_get_sunday_week_of_year  (const GDate *date);

Returns the week of the year during which this date falls, if weeks are understood to being on Sunday. The date must be valid. Can return 0 if the day is before the first Sunday of the year.

`date` : | a GDate.
Returns : | week number.

### g_date_get_sunday_weeks_in_year ()

    guint8      g_date_get_sunday_weeks_in_year (GDateYear year);

Returns the number of weeks in the year, where weeks are taken to start on Sunday. Will be 52 or 53. The date must be valid. (Years always have 52 7-day periods, plus 1 or 2 extra days depending on whether it's a leap year. This function is basically telling you how many Sundays are in the year, i.e. there are 53 Sundays if one of the extra days happens to be a Sunday.)

`year` : | year to count weeks in.
Returns : | number of weeks.

### g_date_strftime ()

    gsize       g_date_strftime                 (gchar *s,
                                                 gsize slen,
                                                 const gchar *format,
                                                 const GDate *date);

Generates a printed representation of the date, in a locale-specific way. Works just like the standard C ``strftime()`` function, but only accepts date-related formats; time-related formats give undefined results. Date must be valid.

`s` : | destination buffer.
`slen` : | buffer size.
`format` : | format string.
`date` : | valid GDate.
Returns : | number of characters written to the buffer, or 0 the buffer was too small.

### g_date_to_struct_tm ()

    void        g_date_to_struct_tm             (const GDate *date,
                                                 struct tm *tm);

Fills in the date-related bits of a struct tm using the `date` value. Initializes the non-date parts with something sane but meaningless.

`date` : | a GDate to set the struct tm from.
`tm` : | struct tm to fill.

### g_date_valid ()

    gboolean    g_date_valid                    (const GDate *date);

Returns `TRUE` if the GDate represents an existing day. The date must not contain garbage; it should have been initialized with `g_date_clear()` if it wasn't allocated by one of the `g_date_new()` variants.

`date` : | a GDate to check.
Returns : | Whether the date is valid.

### g_date_valid_day ()

    gboolean    g_date_valid_day                (GDateDay day);

Returns `TRUE` if the day of the month is valid (a day is valid if it's between 1 and 31 inclusive).

`day` : | day to check.
Returns : | `TRUE` if the day is valid.

### g_date_valid_month ()

    gboolean    g_date_valid_month              (GDateMonth month);

Returns `TRUE` if the month value is valid. The 12 GDateMonth enumeration values are the only valid months.

`month` : | month.
Returns : | `TRUE` if the month is valid.

### g_date_valid_year ()

    gboolean    g_date_valid_year               (GDateYear year);

Returns `TRUE` if the year is valid. Any year greater than 0 is valid, though there is a 16-bit limit to what GDate will understand.

`year` : | year.
Returns : | `TRUE` if the year is valid.

### g_date_valid_dmy ()

    gboolean    g_date_valid_dmy                (GDateDay day,
                                                 GDateMonth month,
                                                 GDateYear year);

Returns `TRUE` if the day-month-year triplet forms a valid, existing day in the range of days GDate understands (Year 1 or later, no more than a few thousand years in the future).

`day` : | day.
`month` : | month.
`year` : | year.
Returns : | `TRUE` if the date is a valid one.

### g_date_valid_julian ()

    gboolean    g_date_valid_julian             (guint32 julian_date);

Returns `TRUE` if the Julian day is valid. Anything greater than zero is basically a valid Julian, though there is a 32-bit limit.

`julian_date` : | Julian day to check.
Returns : | `TRUE` if the Julian day is valid.

### g_date_valid_weekday ()

    gboolean    g_date_valid_weekday            (GDateWeekday weekday);

Returns `TRUE` if the weekday is valid. The 7 GDateWeekday enumeration values are the only valid weekdays.

`weekday` : | weekday.
Returns : | `TRUE` if the weekday is valid.