boost_1_45_0/libs/date_time/xmldoc/snap_to_details.xml

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     Subject to the Boost Software License, Version 1.0. 
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<para>
<anchor id="snap_to_details" />
<bridgehead renderas="sect4">Reversibility of Operations Pitfall</bridgehead>
<para>A natural expectation when adding a number of months to a date, and then subtracting the same number of months, is to end up exactly where you started. This is most often the result the <code>date_time</code> library provides but there is one significant exception: The snap-to-end-of-month behavior implemented by the <link linkend="additional_duration_types">months</link> duration type. The <link linkend="additional_duration_types">months</link> duration type may provide unexpected results when the starting day is the 28th, 29th, or 30th in a 31 day month. The <link linkend="iterators_intro">month_iterator</link> is not affected by this issue and is therefore included in the examples to illustrate a possible alternative.
</para>

<para>When the starting date is in the middle of a month, adding or subtracting any number of months will result in a date that is the same day of month (e.g. if you start on the 15th, you will end on the 15th). When a date is the last day of the month, adding or subtracting any number of months will give a result that is also the last day of the month (e.g if you start on Jan 31st, you will land on: Feb 28th, Mar 31st, etc).
  <programlisting>
    // using months duration type
    date d(2005, Nov, 30); // last day of November
    d + months(1); // result is last day of December "2005-Dec-31"
    d - months(1); // result is last day of October "2005-Oct-31"

    // using month_iterator
    month_iterator itr(d); // last day of November
    ++itr; // result is last day of December "2005-Dec-31"
    --itr; // back to original starting point "2005-Nov-30"
    --itr; // last day of October "2005-Oct-31"
  </programlisting>
</para>

<para>If the start date is the 28th, 29th, or 30th in a 31 day month, the result of adding or subtracting a month may result in the snap-to-end-of-month behavior kicking in unexpectedly. This would cause the final result to be different than the starting date.
  <programlisting>
    // using months duration type
    date d(2005, Nov, 29);
    d += months(1); // "2005-Dec-29"
    d += months(1); // "2006-Jan-29"
    d += months(1); // "2006-Feb-28" --> snap-to-end-of-month behavior kicks in
    d += months(1); // "2006-Mar-31" --> unexpected result
    d -= months(4); // "2005-Nov-30" --> unexpected result, not where we started

    // using month_iterator
    month_iterator itr(date(2005, Dec, 30));
    ++itr; // "2006-Jan-30" --> ok
    ++itr; // "2006-Feb-28" --> snap-to DOES NOT kick in 
    ++itr; // "2006-Mar-30" --> ok
    --itr; // "2006-Feb-28" --> ok
    --itr; // "2006-Jan-30" --> ok
    --itr; // "2005-Dec-30" --> ok, back where we started
  </programlisting>
</para>

<para>The additional duration types (<code>months</code>, <code>years</code>, and <code>weeks</code>) are provided as a convenience and can be easily removed to insure this pitfall never occurs. To remove these types simply undefine BOOST_DATE_TIME_OPTIONAL_GREGORIAN_TYPES.</para>
</para>