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  <header>
    <copyright>
      <year>1996</year><year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.
    
    </legalnotice>

    <title>calendar</title>
    <prepared>Peter H&ouml;gfeldt</prepared>
    <docno></docno>
    <date>1996-11-05</date>
    <rev>B</rev>
  </header>
  <module>calendar</module>
  <modulesummary>Local and universal time, day of the week, date and time
    conversions.</modulesummary>
  <description>
    <p>This module provides computation of local and universal time,
      day of the week, and many time conversion functions.</p>
    <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>
    <p>The time functions <c>local_time/0</c> and
      <c>universal_time/0</c> in this module both return date
      and time. The is because separate functions for date
      and time can result in a date/time combination that is displaced
      by 24 hours. This occurs if one of the functions is called
      before midnight, and the other after midnight. This problem also
      applies to the Erlang BIFs <c>date/0</c> and <c>time/0</c>, and
      their use is strongly discouraged if a reliable date/time stamp
      is required.</p>
    <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>
    <p>The Gregorian calendar in this module is extended back to year 0.
      For a given date, the <em>gregorian days</em> is the number of
      days up to and including the date specified. Similarly,
      the <em>gregorian seconds</em> for a specified date and time is
      the number of seconds up to and including the specified date
      and time.</p>
    <p>For computing differences between epochs in time, use
      the functions counting gregorian days or seconds. If epochs are
      specified as local time, they must be converted to universal time
      to get the correct value of the elapsed time between epochs.
      Use of function <c>time_difference/2</c> is discouraged.</p>
    <p>Different definitions exist for the week of the year.
      This module contains a week of the year implementation
      conforming to the ISO 8601 standard. As the week number for a
      specified date can fall on the previous, the current, or on the next
      year, it is important to specify both the year and the week number.
      Functions <c>iso_week_number/0</c> and <c>iso_week_number/1</c>
      return a tuple of the year and the week number.</p>
  </description>

  <datatypes>
    <datatype>
      <name name="datetime"/>
    </datatype>
    <datatype>
      <name name="datetime1970"/>
    </datatype>
    <datatype>
      <name name="date"/>
    </datatype>
    <datatype>
      <name name="year"/>
      <desc><p>Year cannot be abbreviated. For example, 93 denotes year
        93, not 1993. The valid range depends on the underlying operating
        system. The date tuple must denote a valid date.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="year1970"/>
    </datatype>
    <datatype>
      <name name="month"/>
    </datatype>
    <datatype>
      <name name="day"/>
    </datatype>
    <datatype>
      <name name="time"/>
    </datatype>
    <datatype>
      <name name="hour"/>
    </datatype>
    <datatype>
      <name name="minute"/>
    </datatype>
    <datatype>
      <name name="second"/>
    </datatype>
    <datatype>
      <name name="daynum"/>
    </datatype>
    <datatype>
      <name name="ldom"/>
    </datatype>
    <datatype>
      <name name="yearweeknum"/>
    </datatype>
    <datatype>
      <name name="weeknum"/>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="date_to_gregorian_days" arity="1"/>
      <name name="date_to_gregorian_days" arity="3"/>
      <fsummary>Compute the number of days from year 0 up to the specified
        date.</fsummary>
      <type variable="Date" name_i="1"/>
      <type variable="Year"/>
      <type variable="Month"/>
      <type variable="Day"/>
      <desc>
        <p>Computes the number of gregorian days starting
          with year 0 and ending at the specified date.</p>
      </desc>
    </func>

    <func>
      <name name="datetime_to_gregorian_seconds" arity="1"/>
      <fsummary>Compute the number of seconds from year 0 up to the specified
        date and time.</fsummary>
      <desc>
        <p>Computes the number of gregorian seconds starting
          with year 0 and ending at the specified date and time.</p>
      </desc>
    </func>

    <func>
      <name name="day_of_the_week" arity="1"/>
      <name name="day_of_the_week" arity="3"/>
      <fsummary>Compute the day of the week.</fsummary>
      <type variable="Date" name_i="1"/>
      <type variable="Year"/>
      <type variable="Month"/>
      <type variable="Day"/>
      <desc>
        <p>Computes the day of the week from the specified
          <c><anno>Year</anno></c>, <c><anno>Month</anno></c>, and
          <c><anno>Day</anno></c>. Returns the day of the week as
          <c>1</c>: Monday, <c>2</c>: Tuesday, and so on.</p>
      </desc>
    </func>

    <func>
      <name name="gregorian_days_to_date" arity="1"/>
      <fsummary>Compute the date from the number of gregorian days.</fsummary>
      <desc>
        <p>Computes the date from the specified number of gregorian days.</p>
      </desc>
    </func>

    <func>
      <name name="gregorian_seconds_to_datetime" arity="1"/>
      <fsummary>Compute the date and time from the number of gregorian seconds.
      </fsummary>
      <desc>
        <p>Computes the date and time from the specified
          number of gregorian seconds.</p>
      </desc>
    </func>

    <func>
      <name name="is_leap_year" arity="1"/>
      <fsummary>Check if the year is a leap year.</fsummary>
      <desc>
        <p>Checks if the specified year is a leap year.</p>
      </desc>
    </func>

    <func>
      <name name="iso_week_number" arity="0"/>
      <fsummary>Compute the ISO week number for the actual date.</fsummary>
      <desc>
        <p>Returns tuple <c>{Year, WeekNum}</c> representing
          the ISO week number for the actual date. To determine the
          actual date, use function
          <seealso marker="#local_time/0"><c>local_time/0</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="iso_week_number" arity="1"/>
      <fsummary>Compute the ISO week number for the specified date.</fsummary>
      <desc>
        <p>Returns tuple <c>{Year, WeekNum}</c> representing
          the ISO week number for the specified date.</p>
      </desc>
    </func>

    <func>
      <name name="last_day_of_the_month" arity="2"/>
      <fsummary>Compute the number of days in a month.</fsummary>
      <desc>
        <p>Computes the number of days in a month.</p>
      </desc>
    </func>

    <func>
      <name name="local_time" arity="0"/>
      <fsummary>Compute local time.</fsummary>
      <desc>
        <p>Returns the local time reported by
          the underlying operating system.</p>
      </desc>
    </func>

    <func>
      <name name="local_time_to_universal_time" arity="1"/>
      <fsummary>Convert from local time to universal time (deprecated).
      </fsummary>
      <desc>
        <p>Converts from local time to Universal Coordinated Time (UTC).
          <c><anno>DateTime1</anno></c> must refer to a local
          date after Jan 1, 1970.</p>
        <warning>
          <p>This function is deprecated. Use
            <seealso marker="#local_time_to_universal_time_dst/1">
            <c>local_time_to_universal_time_dst/1</c></seealso>
            instead, as it gives a more correct and complete result.
            Especially for
            the period that does not exist, as it is skipped during
            the switch <em>to</em> daylight saving time, this function
            still returns a result.</p>
        </warning>
      </desc>
    </func>

    <func>
      <name name="local_time_to_universal_time_dst" arity="1"/>
      <fsummary>Convert from local time to universal time(s).</fsummary>
      <desc>
        <p>Converts from local time to Universal Coordinated Time (UTC).
          <c><anno>DateTime1</anno></c> must refer to a local
          date after Jan 1, 1970.</p>
        <p>The return value is a list of 0, 1, or 2 possible UTC times:</p>
        <taglist>
          <tag><c>[]</c></tag>
          <item>
            <p>For a local <c>{Date1, Time1}</c> during the period that
              is skipped when switching <em>to</em> daylight saving
              time, there is no corresponding UTC, as the local time
              is illegal (it has never occured).</p>
          </item>
          <tag><c>[DstDateTimeUTC, DateTimeUTC]</c></tag>
          <item>
            <p>For a local <c>{Date1, Time1}</c> during the period that
              is repeated when switching <em>from</em> daylight saving
              time, two corresponding UTCs exist; one for the first
              instance of the period when daylight saving time is still
              active, and one for the second instance.</p>
          </item>
          <tag><c>[DateTimeUTC]</c></tag>
          <item>
            <p>For all other local times only one corresponding UTC exists.</p>
          </item>
        </taglist>
      </desc>
    </func>

    <func>
      <name name="now_to_datetime" arity="1"/>
      <fsummary>Convert now to date and time.</fsummary>
      <desc>
        <p>Returns Universal Coordinated Time (UTC)
          converted from the return value from
          <seealso marker="erts:erlang#timestamp/0"><c>erlang:timestamp/0</c></seealso>.
        </p>
      </desc>
    </func>

    <func>
      <name name="now_to_local_time" arity="1"/>
      <fsummary>Convert now to local date and time.</fsummary>
      <desc>
        <p>Returns local date and time converted from the return value from
        <seealso marker="erts:erlang#timestamp/0"><c>erlang:timestamp/0</c></seealso>.
        </p>
      </desc>
    </func>

    <func>
      <name name="now_to_universal_time" arity="1"/>
      <fsummary>Convert now to date and time.</fsummary>
      <desc>
        <p>Returns Universal Coordinated Time (UTC)
          converted from the return value from
          <seealso marker="erts:erlang#timestamp/0"><c>erlang:timestamp/0</c></seealso>.
        </p>
      </desc>
    </func>

    <func>
      <name name="seconds_to_daystime" arity="1"/>
      <fsummary>Compute days and time from seconds.</fsummary>
      <desc>
        <p>Converts a specified number of seconds into days, hours, minutes,
          and seconds. <c><anno>Time</anno></c> is always non-negative, but
          <c><anno>Days</anno></c> is negative if argument
          <c><anno>Seconds</anno></c> is.</p>
      </desc>
    </func>

    <func>
      <name name="seconds_to_time" arity="1"/>
      <fsummary>Compute time from seconds.</fsummary>
      <type name="secs_per_day"/>
      <desc>
        <p>Computes the time from the specified number of seconds.
          <c><anno>Seconds</anno></c> must be less than the number of
          seconds per day (86400).</p>
      </desc>
    </func>

    <func>
      <name name="time_difference" arity="2"/>
      <fsummary>Compute the difference between two times (deprecated).
      </fsummary>
      <desc>
        <p>Returns the difference between two <c>{Date, Time}</c> tuples.
          <c><anno>T2</anno></c> is to refer to an epoch later
          than <c><anno>T1</anno></c>.</p>
        <warning>
          <p>This function is obsolete. Use the conversion functions for
            gregorian days and seconds instead.</p>
        </warning>
      </desc>
    </func>

    <func>
      <name name="time_to_seconds" arity="1"/>
      <fsummary>Compute the number of seconds since midnight up to the
        specified time.</fsummary>
      <type name="secs_per_day"/>
      <desc>
        <p>Returns the number of seconds since midnight
          up to the specified time.</p>
      </desc>
    </func>

    <func>
      <name name="universal_time" arity="0"/>
      <fsummary>Compute universal time.</fsummary>
      <desc>
        <p>Returns the Universal Coordinated Time (UTC)
          reported by the underlying operating system. Returns local time if
          universal time is unavailable.</p>
      </desc>
    </func>

    <func>
      <name name="universal_time_to_local_time" arity="1"/>
      <fsummary>Convert from universal time to local time.</fsummary>
      <desc>
        <p>Converts from Universal Coordinated Time (UTC) to local time.
          <c><anno>DateTime</anno></c> must refer to a date after Jan 1, 1970.
        </p>
      </desc>
    </func>

    <func>
      <name name="valid_date" arity="1"/>
      <name name="valid_date" arity="3"/>
      <fsummary>Check if a date is valid</fsummary>
      <type variable="Date" name_i="1"/>
      <type variable="Year"/>
      <type variable="Month"/>
      <type variable="Day"/>
      <desc>
        <p>This function checks if a date is a valid.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Leap Years</title>
    <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
      one of the following rules is valid:</p>
    <list type="bulleted">
      <item>
        <p>Y is divisible by 4, but not by 100.</p>
      </item>
      <item>
        <p>Y is divisible by 400.</p>
      </item>
    </list>
    <p>Hence, 1996 is a leap year, 1900 is not, but 2000 is.</p>
  </section>

  <section>
    <title>Date and Time Source</title>
    <p>Local time is obtained from the Erlang BIF <c>localtime/0</c>.
      Universal time is computed from the BIF <c>universaltime/0</c>.</p>
    <p>The following fapply:</p>
    <list type="bulleted">
      <item>There are 86400 seconds in a day.</item>
      <item>There are 365 days in an ordinary year.</item>
      <item>There are 366 days in a leap year.</item>
      <item>There are 1461 days in a 4 year period.</item>
      <item>There are 36524 days in a 100 year period.</item>
      <item>There are 146097 days in a 400 year period.</item>
      <item>There are 719528 days between Jan 1, 0 and Jan 1, 1970.</item>
    </list>
  </section>
</erlref>

