php-doc-en/reference/calendar/functions/easter-days.xml

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<refentry xml:id="function.easter-days" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>easter_days</refname>
  <refpurpose>Get number of days after March 21 on which Easter falls for a given year</refpurpose>
 </refnamediv>

 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>int</type><methodname>easter_days</methodname>
   <methodparam choice="opt"><type class="union"><type>int</type><type>null</type></type><parameter>year</parameter><initializer>&null;</initializer></methodparam>
   <methodparam choice="opt"><type>int</type><parameter>mode</parameter><initializer><constant>CAL_EASTER_DEFAULT</constant></initializer></methodparam>
  </methodsynopsis>
  <para>
   Returns the number of days after March 21 on which Easter falls
   for a given year. If no year is specified, the current year is
   assumed.
  </para>
  <para>
   This function can be used instead of
   <function>easter_date</function> to calculate Easter for years
   which fall outside the range of Unix timestamps (i.e. before 1970
   or after 2037).
  </para>
  <para>
   The date of Easter Day was defined by the Council of Nicaea in
   AD325 as the Sunday after the first full moon which falls on or
   after the Spring Equinox.  The Equinox is assumed to always fall
   on 21st March, so the calculation reduces to determining the date
   of the full moon and the date of the following Sunday.  The
   algorithm used here was introduced around the year 532 by
   Dionysius Exiguus.  Under the Julian Calendar (for years before
   1753) a simple 19-year cycle is used to track the phases of the
   Moon.  Under the Gregorian Calendar (for years after 1753 -
   devised by Clavius and Lilius, and introduced by Pope Gregory
   XIII in October 1582, and into Britain and its then colonies in
   September 1752) two correction factors are added to make the
   cycle more accurate.
  </para>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>year</parameter></term>
     <listitem>
      <para>
       The year as a positive number. If omitted or &null;, defaults to the
       current year according to the local time.
      </para>
     </listitem>
    </varlistentry>  
    <varlistentry>
     <term><parameter>mode</parameter></term>
     <listitem>
      <para>
       Allows Easter dates to be calculated based
       on the Gregorian calendar during the years 1582 - 1752 when set to
       <constant>CAL_EASTER_ROMAN</constant>. See the <link linkend="calendar.constants">calendar constants</link> for more valid
       constants.      
       </para>
     </listitem>
    </varlistentry>  
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   The number of days after March 21st that the Easter Sunday
   is in the given <parameter>year</parameter>.
  </para>
 </refsect1>

 <refsect1 role="changelog">
  &reftitle.changelog;
  <informaltable>
   <tgroup cols="2">
    <thead>
     <row>
      <entry>&Version;</entry>
      <entry>&Description;</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry>8.0.0</entry>
      <entry>
       <parameter>year</parameter> is nullable now.
      </entry>
     </row>
    </tbody>
   </tgroup>
  </informaltable>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title><function>easter_days</function> example</title>
    <programlisting role="php">
<![CDATA[
<?php

echo easter_days(1999);        // 14, i.e. April 4
echo easter_days(1492);        // 32, i.e. April 22
echo easter_days(1913);        //  2, i.e. March 23

?>
]]>
    </programlisting>
   </example>
  </para>
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>easter_date</function></member>
   </simplelist>
  </para>
 </refsect1>
</refentry>
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