date.sgml

This GLib version 2.16.1. GLib is the low-level core library that forms the basis for projects such

<!-- ##### 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 day-month-year 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 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>

<!-- ##### SECTION Stability_Level ##### -->


<!-- ##### MACRO G_USEC_PER_SEC ##### -->
<para>
Number of microseconds in one second (1 million). This macro is provided for
code readability.
</para>



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

@tv_sec: seconds
@tv_usec: microseconds

<!-- ##### FUNCTION g_get_current_time ##### -->
<para>
</para>

@result: 


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

@microseconds: number of microseconds to pause


<!-- ##### FUNCTION g_time_val_add ##### -->
<para>

</para>

@time_: 
@microseconds: 


<!-- ##### FUNCTION g_time_val_from_iso8601 ##### -->
<para>

</para>

@iso_date: 
@time_: 
@Returns: 


<!-- ##### FUNCTION g_time_val_to_iso8601 ##### -->
<para>

</para>

@time_: 
@Returns: 


<!-- ##### 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: the Julian representation of the date
@julian: this bit is set if @julian_days is valid
@dmy: this is set if @day, @month and @year are valid
@day: the day of the day-month-year representation of the date, as 
  a number between 1 and 31
@month: the day of the day-month-year representation of the date, as 
  a number between 1 and 12
@year: the day of the day-month-year representation of the date

<!-- ##### TYPEDEF GTime ##### -->
<para>
Simply a replacement for <type>time_t</type>. It has been deprected
since it is <emphasis>not</emphasis> equivalent to <type>time_t</type> 
on 64-bit platforms with a 64-bit <type>time_t</type>. 
Unrelated to #GTimer.
</para>


<para>
Note that <type>GTime</type> is defined to always be a 32bit integer,
unlike <type>time_t</type> which may be 64bit on some systems. 
Therefore, <type>GTime</type> will overflow in the year 2038, and
you cannot use the address of a <type>GTime</type> variable as argument 
to the UNIX time() function. Instead, do the following:
<informalexample>
<programlisting>
time_t ttime;
GTime gtime;

time (&amp;ttime);
gtime = (GTime)ttime;
</programlisting>
</informalexample>
</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: a day
@G_DATE_MONTH: a month
@G_DATE_YEAR: a 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: 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

<!-- ##### 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: 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

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

@Returns: a 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: a newly-allocated #GDate 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: a newly-allocated #GDate initialized with @julian_day


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

@date: pointer to one or more dates to clear
@n_dates: number of dates to clear


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

@date: a #GDate


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

@date: a #GDate
@day: day to set


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

@date: a #GDate
@month: month to set


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

@date: a #GDate
@year: year to set


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

@date: a #GDate
@day: day
@month: month
@y: year


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

@date: a #GDate
@julian_date: Julian day number (days since January 1, Year 1)


<!-- ##### FUNCTION g_date_set_time ##### -->
<para>

</para>

@date: 
@time_: 


<!-- ##### FUNCTION g_date_set_time_t ##### -->
<para>

</para>

@date: 
@timet: 


<!-- ##### FUNCTION g_date_set_time_val ##### -->
<para>

</para>

@date: 
@timeval: 


<!-- ##### FUNCTION g_date_set_parse ##### -->
<para>
Parses a user-inputted string @str, and try to figure out what date it