glib/docs/reference/glib/tmpl/date.sgml

<!-- ##### SECTION Title ##### -->
Date and Time Functions

<!-- ##### SECTION Short_Description ##### -->
calendrical calculations and miscellaneous time stuff.

<!-- ##### SECTION Long_Description ##### -->
<para>
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 <emphasis>day</emphasis>.
</para>

<para>
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).
</para>

<para>
#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
<emphasis>invalid</emphasis>, 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.
</para>

<para>
<emphasis>It is very important to use the API to access the #GDate
struct.</emphasis> Often only the DMY or only the Julian
representation is valid. Sometimes neither is valid. Use the API.
</para>

<para>
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().
</para>

<!-- ##### SECTION See_Also ##### -->
<para>

</para>

<!-- ##### MACRO G_USEC_PER_SEC ##### -->
<para>

</para>



<!-- ##### STRUCT GTimeVal ##### -->
<para>
Represents a precise time, with seconds and microseconds. Same as the
<structname>struct timeval</structname> returned by the
<function>gettimeofday()</function> UNIX call.
</para>

@tv_sec: 
@tv_usec: 

<!-- ##### FUNCTION g_get_current_time ##### -->
<para>
Equivalent to <function>gettimeofday()</function>, but also works on
Win32. Returns the current time.
</para>

@result: 


<!-- ##### FUNCTION g_usleep ##### -->
<para>

</para>

@microseconds: 


<!-- ##### STRUCT GDate ##### -->
<para>
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
<structname>GDate</structname> 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.
</para>

@julian_days: 
@julian: 
@dmy: 
@day: 
@month: 
@year: 

<!-- ##### TYPEDEF GTime ##### -->
<para>
Simply a replacement for time_t. Unrelated to GTimer.
</para>


<!-- ##### ENUM GDateDMY ##### -->
<para>
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.
</para>

@G_DATE_DAY: 
@G_DATE_MONTH: 
@G_DATE_YEAR: 

<!-- ##### TYPEDEF GDateDay ##### -->
<para>
Integer representing a day of the month; between 1 and
31. #G_DATE_BAD_DAY represents an invalid day of the month.
</para>


<!-- ##### ENUM GDateMonth ##### -->
<para>
Enumeration representing a month; values are #G_DATE_JANUARY,
#G_DATE_FEBRUARY, etc. #G_DATE_BAD_MONTH is the "invalid" value.
</para>

@G_DATE_BAD_MONTH: 
@G_DATE_JANUARY: 
@G_DATE_FEBRUARY: 
@G_DATE_MARCH: 
@G_DATE_APRIL: 
@G_DATE_MAY: 
@G_DATE_JUNE: 
@G_DATE_JULY: 
@G_DATE_AUGUST: 
@G_DATE_SEPTEMBER: 
@G_DATE_OCTOBER: 
@G_DATE_NOVEMBER: 
@G_DATE_DECEMBER: 

<!-- ##### TYPEDEF GDateYear ##### -->
<para>
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.
</para>


<!-- ##### ENUM GDateWeekday ##### -->
<para>
Enumeration representing a day of the week; #G_DATE_MONDAY,
#G_DATE_TUESDAY, etc. #G_DATE_BAD_WEEKDAY is an invalid weekday.
</para>

@G_DATE_BAD_WEEKDAY: 
@G_DATE_MONDAY: 
@G_DATE_TUESDAY: 
@G_DATE_WEDNESDAY: 
@G_DATE_THURSDAY: 
@G_DATE_FRIDAY: 
@G_DATE_SATURDAY: 
@G_DATE_SUNDAY: 

<!-- ##### MACRO G_DATE_BAD_DAY ##### -->
<para>
Represents an invalid #GDateDay.
</para>



<!-- ##### MACRO G_DATE_BAD_JULIAN ##### -->
<para>
Represents an invalid Julian day number.
</para>



<!-- ##### MACRO G_DATE_BAD_YEAR ##### -->
<para>
Represents an invalid year.
</para>



<!-- ##### FUNCTION g_date_new ##### -->
<para>
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().
</para>

@Returns: The newly-allocated #GDate


<!-- ##### FUNCTION g_date_new_dmy ##### -->
<para>
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.
</para>

@day: Day of the month
@month: Month of the year
@year: Year
@Returns: Allocated date initialized with @day, @month, and @year


<!-- ##### FUNCTION g_date_new_julian ##### -->
<para>
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.
</para>

@julian_day: Days since January 1, Year 1
@Returns: Allocated date initialized with @julian_day


<!-- ##### FUNCTION g_date_clear ##### -->
<para>
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().
</para>

@date: Pointer to one or more dates to clear
@n_dates: Number of dates to clear


<!-- ##### FUNCTION g_date_free ##### -->
<para>
Free a #GDate returned from g_date_new()
</para>

@date: Date to free


<!-- ##### FUNCTION g_date_set_day ##### -->
<para>
Set the day of the month for a #GDate. If the resulting day-month-year
triplet is invalid, the date will be invalid.
</para>

@date: Date to set the day for
@day: Day to set


<!-- ##### FUNCTION g_date_set_month ##### -->
<para>
Set the month of the year for a #GDate.  If the resulting
 day-month-year triplet is invalid, the date will be invalid.
</para>

@date: Date
@month: Month to set


<!-- ##### FUNCTION g_date_set_year ##### -->
<para>
Set the year for a #GDate.  If the resulting day-month-year triplet is
  invalid, the date will be invalid.
</para>

@date: Date
@year: Year to set


<!-- ##### FUNCTION g_date_set_dmy ##### -->
<para>
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.
</para>

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


<!-- ##### FUNCTION g_date_set_julian ##### -->
<para>
Set the value of a #GDate from a Julian day number. 
</para>

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


<!-- ##### FUNCTION g_date_set_time ##### -->
<para>
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:
<informalexample><programlisting>
  g_date_set_time(date, time(NULL));
</programlisting></informalexample>
</para>

@date: Date to update
@time: #GTime value to set


<!-- ##### FUNCTION g_date_set_parse ##### -->
<para>
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.
</para>

<para>
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).
</para>

@date: Date to fill in
@str: String to parse


<!-- ##### FUNCTION g_date_add_days ##### -->
<para>
Increment a date some number of days. To move forward by weeks, add
weeks*7 days. The date must be valid.
</para>

@date: The date to increment
@n_days: Number of days to move the date forward


<!-- ##### FUNCTION g_date_subtract_days ##### -->
<para>
Move a date some number of days into the past. To move by weeks, just
move by weeks*7 days. Date must be valid.
</para>

@date: Date to decrement
@n_days: Number of days to move


<!-- ##### FUNCTION g_date_add_months ##### -->
<para>
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.
</para>

@date: Date to increment
@n_months: Number of months to move forward


<!-- ##### FUNCTION g_date_subtract_months ##### -->
<para>
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.
</para>

@date: Date to decrement
@n_months: Number of months to move


<!-- ##### FUNCTION g_date_add_years ##### -->
<para>
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.
</para>

@date: Date to increment
@n_years: Number of years to move forward


<!-- ##### FUNCTION g_date_subtract_years ##### -->
<para>
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.
</para>

@date: Date to decrement
@n_years: Number of years to move


<!-- ##### FUNCTION g_date_compare ##### -->
<para>
<function>qsort()</function>-style comparsion function for dates. Both
dates must be valid.
</para>

@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


<!-- ##### FUNCTION g_date_get_day ##### -->
<para>
Return the day of the month; the #GDate must be valid.
</para>

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


<!-- ##### FUNCTION g_date_get_month ##### -->
<para>
Accessor for the month of the year. Date must be valid.
</para>

@date: Date to get the month from
@Returns: A #GDateMonth


<!-- ##### FUNCTION g_date_get_year ##### -->
<para>
Accessor; returns the year of a #GDate. The date must be valid.
</para>

@date: Date
@Returns: Year in which the date falls


<!-- ##### FUNCTION g_date_get_julian ##### -->
<para>
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.
</para>

@date: Date to extract the Julian day from
@Returns: Julian day


<!-- ##### FUNCTION g_date_get_weekday ##### -->
<para>
Returns the day of the week for a #GDate. The date must be valid.
</para>

@date: Date
@Returns: Day of the week as a #GDateWeekday


<!-- ##### FUNCTION g_date_get_day_of_year ##### -->
<para>
Return the day of the year, where Jan 1 is the first day of the
year. Date must be valid.
</para>

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


<!-- ##### FUNCTION g_date_get_days_in_month ##### -->
<para>
Return the number of days in a month, taking leap years into account.
</para>

@month: Month
@year: Year
@Returns: Number of days in @month during the year @year.


<!-- ##### FUNCTION g_date_is_first_of_month ##### -->
<para>
Returns TRUE if the date is on the first of a month. Date must be valid.
</para>

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


<!-- ##### FUNCTION g_date_is_last_of_month ##### -->
<para>
Returns TRUE if the date is the last day of the month. Date must be valid.
</para>

@date: Date to check
@Returns: Boolean, if the date is the last day of the month


<!-- ##### FUNCTION g_date_is_leap_year ##### -->
<para>
Returns TRUE if the year is a leap year
</para>

@year: Year to check
@Returns: Boolean, if the year is a leap year


<!-- ##### FUNCTION g_date_get_monday_week_of_year ##### -->
<para>
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.
</para>

@date: Date to use
@Returns: Week of the year


<!-- ##### FUNCTION g_date_get_monday_weeks_in_year ##### -->
<para>
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.)
</para>

@year: Year
@Returns: Number of Mondays in the year


<!-- ##### FUNCTION g_date_get_sunday_week_of_year ##### -->
<para>
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.
</para>

@date: Date
@Returns: Week number


<!-- ##### FUNCTION g_date_get_sunday_weeks_in_year ##### -->
<para>
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.)
</para>

@year: Year to count weeks in
@Returns: Number of weeks


<!-- ##### FUNCTION g_date_strftime ##### -->
<para>
Generate a printed representation of the date, in a locale-specific
way. Works just like the standard C <function>strftime()</function>
function, but only accepts date-related formats; time-related formats
give undefined results. Date must be valid.
</para>

@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


<!-- ##### FUNCTION g_date_to_struct_tm ##### -->
<para>
Fills in the date-related bits of a <structname>struct tm</structname>
using the @date value. Initializes the non-date parts with something
sane but meaningless.
</para>

@date: Date to set the <structname>struct tm</structname> from
@tm: <structname>struct tm</structname> to fill


<!-- ##### FUNCTION g_date_valid ##### -->
<para>
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.
</para>

@date: Date to check
@Returns: Whether the date is valid.


<!-- ##### FUNCTION g_date_valid_day ##### -->
<para>
Returns TRUE if the day of the month is valid (a day is valid if it's
between 1 and 31 inclusive).
</para>

@day: Day to check.
@Returns: Boolean, whether the day is valid.


<!-- ##### FUNCTION g_date_valid_month ##### -->
<para>
Returns TRUE if the month value is valid. The 12 #GDateMonth
enumeration values are the only valid months.
</para>

@month: Month
@Returns: Boolean, whether the month is valid


<!-- ##### FUNCTION g_date_valid_year ##### -->
<para>
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.
</para>

@year: Year
@Returns: Boolean, whether the year is valid.


<!-- ##### FUNCTION g_date_valid_dmy ##### -->
<para>
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).
</para>

@day: Day
@month: Month
@year: Year
@Returns: Boolean, whether the date is a valid one


<!-- ##### FUNCTION g_date_valid_julian ##### -->
<para>
Returns TRUE if the Julian day is valid. Anything greater than zero is basically a
valid Julian, though there is a 32-bit limit.
</para>

@julian_date: Julian day to check
@Returns: Boolean, whether the Julian day is valid.


<!-- ##### FUNCTION g_date_valid_weekday ##### -->
<para>
Returns TRUE if the weekday is valid. The 7 #GDateWeekday enumeration
values are the only valid weekdays.
</para>

@weekday: Weekday
@Returns: Boolean, whether the weekday is valid.