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<H1>calendar</H1>
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<H3>MODULE</H3>
<DIV CLASS=REFBODY>
calendar
</DIV>

<H3>MODULE SUMMARY</H3>
<DIV CLASS=REFBODY>
Local and universal time, day-of-the-week, date and
time conversions
</DIV>

<H3>DESCRIPTION</H3>
<DIV CLASS=REFBODY>

<P>This module provides computation of local and universal time,
day-of-the-week, and several time conversion functions.
<P>Time is local when it is adjusted in accordance with the current
time zone and daylight saving. Time is universal when it reflects
the time at longitude zero, without any adjustment for daylight
saving. Universal Coordinated Time (UTC) time is also called
Greenwich Mean Time (GMT).
<P>The time functions <CODE>local_time/0</CODE> and
<CODE>universal_time/0</CODE> provided in this module both return date
and time. The reason for this is that separate functions for date
and time may result in a date/time combination which is displaced
by 24 hours. This happens if one of the functions is called
before midnight, and the other after midnight. This problem also
applies to the Erlang BIFs <CODE>date/0</CODE> and <CODE>time/0</CODE>, and
their use is strongly discouraged if a reliable date/time stamp
is required.
<P>All dates conform to the Gregorian calendar. This calendar was
introduced by Pope Gregory XIII in 1582 and was used in all
Catholic countries from this year. Protestant parts of Germany
and the Netherlands adopted it in 1698, England followed in 1752,
and Russia in 1918 (the October revolution of 1917 took place in
November according to the Gregorian calendar).
<P>The Gregorian calendar in this module is extended back to year 0.
For a given date, the <STRONG>gregorian days</STRONG> is the number of
days up to and including the date specified. Similarly,
the <STRONG>gregorian seconds</STRONG> for a given date and time, is
the the number of seconds up to and including the specified date
and time.
<P>For computing differences between epochs in time, use
the functions counting gregorian days or seconds. If epochs are
given as local time, they must be converted to universal time, in
order to get the correct value of the elapsed time between epochs.
Use of the function <CODE>time_difference/2</CODE> is discouraged.
</DIV>

<H3>DATA TYPES</H3>
<DIV CLASS=REFBODY>

<PRE>
date() = {Year, Month, Day}
  Year = int()
  Month = 1..12
  Day = 1..31
Year cannot be abbreviated. Example: 93 denotes year 93, not 1993.
Valid range depends on the underlying OS.
The date tuple must denote a valid date.

time() = {Hour, Minute, Second}
  Hour = 0..23
  Minute = Second = 0..59
    
</PRE>

</DIV>

<H3>EXPORTS</H3>

<P><A NAME="date_to_gregorian_days/1"><STRONG><CODE>date_to_gregorian_days(Date) -&#62; Days</CODE></STRONG></A><BR>
<A NAME="date_to_gregorian_days/3"><STRONG><CODE>date_to_gregorian_days(Year, Month, Day) -&#62; Days</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>Date = date()</CODE></STRONG><BR>
<STRONG><CODE>Days = int()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>This function computes the number of gregorian days starting
         with year 0 and ending at the given date.
</DIV>

<P><A NAME="datetime_to_gregorian_seconds/2"><STRONG><CODE>datetime_to_gregorian_seconds({Date, Time}) -&#62; Seconds
</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>Date = date()</CODE></STRONG><BR>
<STRONG><CODE>Time = time()</CODE></STRONG><BR>
<STRONG><CODE>Seconds = int()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>This function computes the number of gregorian seconds
         starting with year 0 and ending at the given date and time.
        
</DIV>

<P><A NAME="day_of_the_week/1"><STRONG><CODE>day_of_the_week(Date) -&#62; DayNumber</CODE></STRONG></A><BR>
<A NAME="day_of_the_week/3"><STRONG><CODE>day_of_the_week(Year, Month, Day) -&#62; DayNumber</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>Date = date()</CODE></STRONG><BR>
<STRONG><CODE>DayNumber = 1..7</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>This function computes the day of the week given <CODE>Year</CODE>,
         <CODE>Month</CODE> and <CODE>Day</CODE>. The return value denotes the day
         of the week as <CODE>1</CODE>: Monday, <CODE>2</CODE>: Tuesday, and so on.
        
</DIV>

<P><A NAME="gregorian_days_to_date/1"><STRONG><CODE>gregorian_days_to_date(Days) -&#62; Date</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>Days = int()</CODE></STRONG><BR>
<STRONG><CODE>Date = date()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>This function computes the date given the number of
         gregorian days.
</DIV>

<P><A NAME="gregorian_seconds_to_datetime/1"><STRONG><CODE>gregorian_seconds_to_datetime(Seconds) -&#62; {Date, Time}
</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>Seconds = int()</CODE></STRONG><BR>
<STRONG><CODE>Date = date()</CODE></STRONG><BR>
<STRONG><CODE>Time = time()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>This function computes the date and time from the given
         number of gregorian seconds.
</DIV>

<P><A NAME="is_leap_year/1"><STRONG><CODE>is_leap_year(Year) -&#62; bool()</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY>

<P>This function checks if a year is a leap year.
</DIV>

<P><A NAME="last_day_of_the_month/2"><STRONG><CODE>last_day_of_the_month(Year, Month) -&#62; int()</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY>

<P>This function computes the number of days in a month.
</DIV>

<P><A NAME="local_time/0"><STRONG><CODE>local_time() -&#62; {Date, Time}</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>Date = date()</CODE></STRONG><BR>
<STRONG><CODE>Time = time()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>This function returns the local time reported by
         the underlying operating system.
</DIV>

<P><A NAME="local_time_to_universal_time/2"><STRONG><CODE>local_time_to_universal_time({Date1, Time1}) -&#62;
        {Date2, Time2}</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY>

<P>This function converts from local time to Universal 
         Coordinated Time (UTC). <CODE>Date1</CODE> must refer to a local
         date after Jan 1, 1970.
<P>
<TABLE CELLPADDING=4>
  <TR>
    <TD VALIGN=TOP><IMG ALT="Warning!" SRC="warning.gif"></TD>
    <TD>

<P>This function is deprecated. Use
         <CODE>local_time_to_universal_time_dst/1</CODE> instead, as it
         gives a more correct and complete result. Especially for
         the period that does not exist since it gets skipped during
         the switch <STRONG>to</STRONG> daylight saving time, this function
         still returns a result.    </TD>
  </TR>
</TABLE>

</DIV>

<P><A NAME="local_time_to_universal_time_dst/2"><STRONG><CODE>local_time_to_universal_time_dst({Date1, Time1}) -&#62;
        [{Date, Time}]</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>Date1 = Date = date()</CODE></STRONG><BR>
<STRONG><CODE>Time1 = Time = time()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>This function converts from local time to Universal
         Coordinated Time (UTC). <CODE>Date1</CODE> must refer to a local
         date after Jan 1, 1970.
<P>The return value is a list of 0, 1 or 2 possible UTC times:
        
<P>
<DL>

<DT>
<CODE>[]</CODE>
</DT>

<DD>
         For a local <CODE>{Date1, Time1}</CODE> during the period that
         is skipped when switching <STRONG>to</STRONG> daylight saving
         time, there is no corresponding UTC since the local time
         is illegal - it has never happened.<BR>

         
</DD>

<DT>
<CODE>[DstDateTimeUTC, DateTimeUTC]</CODE>
</DT>

<DD>
         For a local <CODE>{Date1, Time1}</CODE> during the period that
         is repeated when switching <STRONG>from</STRONG> daylight saving
         time, there are two corresponding UTCs. One for the first
         instance of the period when daylight saving time is still
         active, and one for the second instance.<BR>

</DD>

<DT>
<CODE>[DateTimeUTC]</CODE>
</DT>

<DD>
         For all other local times there is only one
         corresponding UTC.<BR>

</DD>

</DL>

</DIV>

<P><A NAME="now_to_local_time/1"><STRONG><CODE>now_to_local_time(Now) -&#62; {Date, Time}</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>Now -- see erlang:now/0</CODE></STRONG><BR>
<STRONG><CODE>Date = date()</CODE></STRONG><BR>
<STRONG><CODE>Time = time()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>This function returns local date and time converted from
         the return value from <CODE>erlang:now()</CODE>.
</DIV>

<P><A NAME="now_to_universal_time/1"><STRONG><CODE>now_to_universal_time(Now) -&#62; {Date, Time}</CODE></STRONG></A><BR>
<A NAME="now_to_datetime/1"><STRONG><CODE>now_to_datetime(Now) -&#62; {Date, Time}</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>Now -- see erlang:now/0</CODE></STRONG><BR>
<STRONG><CODE>Date = date()</CODE></STRONG><BR>
<STRONG><CODE>Time = time()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>This function returns Universal Coordinated Time (UTC)
         converted from the return value from <CODE>erlang:now()</CODE>.
</DIV>

<P><A NAME="seconds_to_daystime/1"><STRONG><CODE>seconds_to_daystime(Seconds) -&#62; {Days, Time}</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>Seconds = Days = int()</CODE></STRONG><BR>
<STRONG><CODE>Time = time()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>This function transforms a given number of seconds into days,
         hours, minutes, and seconds. The <CODE>Time</CODE> part is always
         non-negative, but <CODE>Days</CODE> is negative if the argument
         <CODE>Seconds</CODE> is.
</DIV>

<P><A NAME="seconds_to_time/1"><STRONG><CODE>seconds_to_time(Seconds) -&#62; Time</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>Seconds = int() &#60; 86400</CODE></STRONG><BR>
<STRONG><CODE>Time = time()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>This function computes the time from the given number of
         seconds. <CODE>Seconds</CODE> must be less than the number of
         seconds per day (86400).
</DIV>

<P><A NAME="time_difference/2"><STRONG><CODE>time_difference(T1, T2) -&#62; {Days, Time}</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY>

<P>This function returns the difference between two <CODE>{Date,
         Time}</CODE> tuples. <CODE>T2</CODE> should refer to an epoch later
         than <CODE>T1</CODE>.
<P>
<TABLE CELLPADDING=4>
  <TR>
    <TD VALIGN=TOP><IMG ALT="Warning!" SRC="warning.gif"></TD>
    <TD>

<P>This function is obsolete. Use the conversion functions for
         gregorian days and seconds instead.    </TD>
  </TR>
</TABLE>

</DIV>

<P><A NAME="time_to_seconds/1"><STRONG><CODE>time_to_seconds(Time) -&#62; Seconds</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>Time = time()</CODE></STRONG><BR>
<STRONG><CODE>Seconds = int()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>This function computes the number of seconds since midnight
         up to the specified time.
</DIV>

<P><A NAME="universal_time/0"><STRONG><CODE>universal_time() -&#62; {Date, Time}</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>Date = date()</CODE></STRONG><BR>
<STRONG><CODE>Time = time()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>This function returns the Universal Coordinated Time (UTC)
         reported by the underlying operating system. Local time is
         returned if universal time is not available.
</DIV>

<P><A NAME="universal_time_to_local_time/2"><STRONG><CODE>universal_time_to_local_time({Date1, Time1}) -&#62;
        {Date2, Time2}</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>Date1 = Date2 = date()</CODE></STRONG><BR>
<STRONG><CODE>Time1 = Time2 = time()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>This function converts from Universal Coordinated Time (UTC)
         to local time. <CODE>Date1</CODE> must refer to a date after Jan 1, 
         1970.
</DIV>

<P><A NAME="valid_date/1"><STRONG><CODE>valid_date(Date) -&#62; bool()</CODE></STRONG></A><BR>
<A NAME="valid_date/3"><STRONG><CODE>valid_date(Year, Month, Day) -&#62; bool()</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>Date = date()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>This function checks if a date is a valid.
</DIV>

<H3>Leap Years</H3>
<DIV CLASS=REFBODY>

<P>The notion that every fourth year is a leap year is not
completely true. By the Gregorian rule, a year Y is a leap year if
either of the following rules is valid:
<P>
<UL>

<LI>
Y is divisible by 4, but not by 100; or<BR>

</LI>


<LI>
Y is divisible by 400.<BR>

</LI>


</UL>

<P>Accordingly, 1996 is a leap year, 1900 is not, but 2000 is.
</DIV>

<H3>Date and Time Source</H3>
<DIV CLASS=REFBODY>

<P>Local time is obtained from the Erlang BIF <CODE>localtime/0</CODE>.
Universal time is computed from the BIF <CODE>universaltime/0</CODE>.

<P>The following facts apply:
<P>
<UL>

<LI>
there are 86400 seconds in a day
</LI>


<LI>
there are 365 days in an ordinary year
</LI>


<LI>
there are 366 days in a leap year
</LI>


<LI>
there are 1461 days in a 4 year period
</LI>


<LI>
there are 36524 days in a 100 year period
</LI>


<LI>
there are 146097 days in a 400 year period
</LI>


<LI>
there are 719528 days between Jan 1, 0 and Jan 1, 1970.

</LI>


</UL>

</DIV>

<H3>AUTHORS</H3>
<DIV CLASS=REFBODY>
Peter Hgfeldt - support@erlang.ericsson.se<BR>

</DIV>
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<HR>
<SMALL>stdlib 1.14.2<BR>
Copyright &copy; 1991-2006
<A HREF="http://www.erlang.se">Ericsson AB</A><BR>
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