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<a name="Relative-items-in-date-strings"></a>
<div class="header">
<p>
Next: <a href="Pure-numbers-in-date-strings.html#Pure-numbers-in-date-strings" accesskey="n" rel="next">Pure numbers in date strings</a>, Previous: <a href="Day-of-week-items.html#Day-of-week-items" accesskey="p" rel="prev">Day of week items</a>, Up: <a href="Date-Input-Formats.html#Date-Input-Formats" accesskey="u" rel="up">Date Input Formats</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Relative-items-in-date-strings-1"></a>
<h3 class="section">B.6 Relative items in date strings</h3>
<a name="index-relative-items-in-date-strings"></a>
<a name="index-displacement-of-dates"></a>
<p><em>Relative items</em> adjust a date (or the current date if none) forward
or backward. The effects of relative items accumulate. Here are some
examples:
</p>
<div class="example">
<pre class="example">1 year
1 year ago
3 years
2 days
</pre></div>
<a name="index-year-in-date-strings"></a>
<a name="index-month-in-date-strings"></a>
<a name="index-fortnight-in-date-strings"></a>
<a name="index-week-in-date-strings"></a>
<a name="index-day-in-date-strings"></a>
<a name="index-hour-in-date-strings"></a>
<a name="index-minute-in-date-strings"></a>
<p>The unit of time displacement may be selected by the string ‘<samp>year</samp>’
or ‘<samp>month</samp>’ for moving by whole years or months. These are fuzzy
units, as years and months are not all of equal duration. More precise
units are ‘<samp>fortnight</samp>’ which is worth 14 days, ‘<samp>week</samp>’ worth 7
days, ‘<samp>day</samp>’ worth 24 hours, ‘<samp>hour</samp>’ worth 60 minutes,
‘<samp>minute</samp>’ or ‘<samp>min</samp>’ worth 60 seconds, and ‘<samp>second</samp>’ or
‘<samp>sec</samp>’ worth one second. An ‘<samp>s</samp>’ suffix on these units is
accepted and ignored.
</p>
<a name="index-ago-in-date-strings"></a>
<p>The unit of time may be preceded by a multiplier, given as an optionally
signed number. Unsigned numbers are taken as positively signed. No
number at all implies 1 for a multiplier. Following a relative item by
the string ‘<samp>ago</samp>’ is equivalent to preceding the unit by a
multiplier with value <em>-1</em>.
</p>
<a name="index-day-in-date-strings-1"></a>
<a name="index-tomorrow-in-date-strings"></a>
<a name="index-yesterday-in-date-strings"></a>
<p>The string ‘<samp>tomorrow</samp>’ is worth one day in the future (equivalent
to ‘<samp>day</samp>’), the string ‘<samp>yesterday</samp>’ is worth
one day in the past (equivalent to ‘<samp>day ago</samp>’).
</p>
<a name="index-now-in-date-strings"></a>
<a name="index-today-in-date-strings"></a>
<a name="index-this-in-date-strings"></a>
<p>The strings ‘<samp>now</samp>’ or ‘<samp>today</samp>’ are relative items corresponding
to zero-valued time displacement, these strings come from the fact
a zero-valued time displacement represents the current time when not
otherwise changed by previous items. They may be used to stress other
items, like in ‘<samp>12:00 today</samp>’. The string ‘<samp>this</samp>’ also has
the meaning of a zero-valued time displacement, but is preferred in
date strings like ‘<samp>this thursday</samp>’.
</p>
<p>When a relative item causes the resulting date to cross a boundary
where the clocks were adjusted, typically for daylight saving time,
the resulting date and time are adjusted accordingly.
</p>
<p>The fuzz in units can cause problems with relative items. For
example, ‘<samp>2003-07-31 -1 month</samp>’ might evaluate to 2003-07-01,
because 2003-06-31 is an invalid date. To determine the previous
month more reliably, you can ask for the month before the 15th of the
current month. For example:
</p>
<div class="example">
<pre class="example">$ date -R
Thu, 31 Jul 2003 13:02:39 -0700
$ date --date='-1 month' +'Last month was %B?'
Last month was July?
$ date --date="$(date +%Y-%m-15) -1 month" +'Last month was %B!'
Last month was June!
</pre></div>
<p>Also, take care when manipulating dates around clock changes such as
daylight saving leaps. In a few cases these have added or subtracted
as much as 24 hours from the clock, so it is often wise to adopt
universal time by setting the <code>TZ</code> environment variable to
‘<samp>UTC0</samp>’ before embarking on calendrical calculations.
</p>
<hr>
<div class="header">
<p>
Next: <a href="Pure-numbers-in-date-strings.html#Pure-numbers-in-date-strings" accesskey="n" rel="next">Pure numbers in date strings</a>, Previous: <a href="Day-of-week-items.html#Day-of-week-items" accesskey="p" rel="prev">Day of week items</a>, Up: <a href="Date-Input-Formats.html#Date-Input-Formats" accesskey="u" rel="up">Date Input Formats</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
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