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# Date and Time Functions

## Name

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

## Synopsis

    #include <glib.h>

    struct      GTimeVal;
    void        g_get_current_time              (GTimeVal *result);

    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_compare                  (GDate *lhs,
                                                 GDate *rhs);

    GDateDay    g_date_day                      (GDate *date);
    GDateMonth  g_date_month                    (GDate *date);
    GDateYear   g_date_year                     (GDate *date);
    guint32     g_date_julian                   (GDate *date);
    GDateWeekday g_date_weekday                 (GDate *date);
    guint       g_date_day_of_year              (GDate *date);

    guint8      g_date_days_in_month            (GDateMonth month,
                                                 GDateYear year);
    gboolean    g_date_is_first_of_month        (GDate *date);
    gboolean    g_date_is_last_of_month         (GDate *date);
    gboolean    g_date_is_leap_year             (GDateYear year);
    guint       g_date_monday_week_of_year      (GDate *date);
    guint8      g_date_monday_weeks_in_year     (GDateYear year);
    guint       g_date_sunday_week_of_year      (GDate *date);
    guint8      g_date_sunday_weeks_in_year     (GDateYear year);

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

    gboolean    g_date_valid                    (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 DMY 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

### 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.

### g_get_current_time ()

    void        g_get_current_time              (GTimeVal *result);

Equivalent to `gettimeofday()`, but also works on Win32. Returns the current time.

`result` : |

### 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.

### 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_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);

Allocate a GDate and initialize 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 : | The 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 : | Allocated date 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 : | Allocated date initialized with `julian_day`

### g_date_clear ()

    void        g_date_clear                    (GDate *date,
                                                 guint n_dates);

Initialize 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);

Free a GDate returned from g_date_new()

`date` : | Date to free

### g_date_set_day ()

    void        g_date_set_day                  (GDate *date,
                                                 GDateDay day);

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

`date` : | Date to set the day for
`day` : | Day to set

### g_date_set_month ()

    void        g_date_set_month                (GDate *date,
                                                 GDateMonth month);

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

`date` : | Date
`month` : | Month to set

### g_date_set_year ()

    void        g_date_set_year                 (GDate *date,
                                                 GDateYear year);

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

`date` : | Date
`year` : | Year to set

### g_date_set_dmy ()

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

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

`date` : | Date to set the value of
`day` : | Day
`month` : | Month
`y` : | Year

### g_date_set_julian ()

    void        g_date_set_julian               (GDate *date,
                                                 guint32 julian_date);

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

`date` : | Date to set
`julian_date` : | Julian day number (days since January 1, Year 1)

### g_date_set_time ()

    void        g_date_set_time                 (GDate *date,
                                                 GTime time);

Set 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` : | Date to update
`time` : | GTime value to set

### g_date_set_parse ()

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

Parse 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` : | Date to fill in
`str` : | String to parse

### g_date_add_days ()

    void        g_date_add_days                 (GDate *date,
                                                 guint n_days);

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

`date` : | The date 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);

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

`date` : | Date to decrement
`n_days` : | Number of days to move

### g_date_add_months ()

    void        g_date_add_months               (GDate *date,
                                                 guint n_months);

Increment 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` : | Date to increment
`n_months` : | Number of months to move forward

### g_date_subtract_months ()

    void        g_date_subtract_months          (GDate *date,
                                                 guint n_months);

Move 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. Date must be valid.

`date` : | Date to decrement
`n_months` : | Number of months to move

### g_date_add_years ()

    void        g_date_add_years                (GDate *date,
                                                 guint n_years);

Increment 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` : | Date to increment
`n_years` : | Number of years to move forward

### g_date_subtract_years ()

    void        g_date_subtract_years           (GDate *date,
                                                 guint n_years);

Move 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. Date must be valid.

`date` : | Date to decrement
`n_years` : | Number of years to move

### g_date_compare ()

    gint        g_date_compare                  (GDate *lhs,
                                                 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_day ()

    GDateDay    g_date_day                      (GDate *date);

Return the day of the month; the GDate must be valid.

`date` : | Date to extract the day of the month from
Returns : | Day of the month

### g_date_month ()

    GDateMonth  g_date_month                    (GDate *date);

Accessor for the month of the year. Date must be valid.

`date` : | Date to get the month from
Returns : | A GDateMonth

### g_date_year ()

    GDateYear   g_date_year                     (GDate *date);

Accessor; returns the year of a GDate. The date must be valid.

`date` : | Date
Returns : | Year in which the date falls

### g_date_julian ()

    guint32     g_date_julian                   (GDate *date);

Accessor, 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. Date must be valid.

`date` : | Date to extract the Julian day from
Returns : | Julian day

### g_date_weekday ()

    GDateWeekday g_date_weekday                 (GDate *date);

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

`date` : | Date
Returns : | Day of the week as a GDateWeekday

### g_date_day_of_year ()

    guint       g_date_day_of_year              (GDate *date);

Return the day of the year, where Jan 1 is the first day of the year. Date must be valid.

`date` : | Date to extract day of year from
Returns : | Day of the year

### g_date_days_in_month ()

    guint8      g_date_days_in_month            (GDateMonth month,
                                                 GDateYear year);

Return 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 `year`.

### g_date_is_first_of_month ()

    gboolean    g_date_is_first_of_month        (GDate *date);

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

`date` : | Date to check
Returns : | Boolean, if the date is the first of the month

### g_date_is_last_of_month ()

    gboolean    g_date_is_last_of_month         (GDate *date);

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

`date` : | Date to check
Returns : | Boolean, 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 : | Boolean, if the year is a leap year

### g_date_monday_week_of_year ()

    guint       g_date_monday_week_of_year      (GDate *date);

Return 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. Date must be valid.

`date` : | Date to use
Returns : | Week of the year

### g_date_monday_weeks_in_year ()

    guint8      g_date_monday_weeks_in_year     (GDateYear year);

Return the number of weeks in the year, where weeks are taken to start on Monday. Will be 52 or 53. 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` : | Year
Returns : | Number of Mondays in the year

### g_date_sunday_week_of_year ()

    guint       g_date_sunday_week_of_year      (GDate *date);

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

`date` : | Date
Returns : | Week number

### g_date_sunday_weeks_in_year ()

    guint8      g_date_sunday_weeks_in_year     (GDateYear year);

Return the number of weeks in the year, where weeks are taken to start on Sunday. Will be 52 or 53. 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,
                                                 GDate *date);

Generate 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` : | Max 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             (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` : | Date to set the struct tm from
`tm` : | struct tm to fill

### g_date_valid ()

    gboolean    g_date_valid                    (GDate *date);

Returns TRUE if the GDate represents an existing day. GDate 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` : | Date 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 : | Boolean, whether 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 : | Boolean, whether 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 : | Boolean, whether 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 : | Boolean, whether 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 : | Boolean, whether 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 : | Boolean, whether the weekday is valid.

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