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<li><a href="#Manipulating_Dates">Manipulating Dates</a></li>
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  <li><strong>05-dates.md</strong></li>
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<h1>Topic <code>05-dates.md</code></h1>

    <h2>Date and Time</h2>

<p><a id="date"></a></p>

<p><a name="Manipulating_Dates"></a></p>

<h3>Manipulating Dates</h3>

<p>The <a href="../modules/pl.Date.html#">Date</a>  class provides a simplified way to work with <a href="http://www.lua.org/pil/22.1.html">date and time</a> in Lua; it leans heavily on the functions <a href="http://www.lua.org/manual/5.1/manual.html#pdf-os.date">os.date</a>  and <a href="http://www.lua.org/manual/5.1/manual.html#pdf-os.time">os.time</a> .</p>

<p>A <a href="../modules/pl.Date.html#">Date</a>  object can be constructed from a table, just like with <a href="http://www.lua.org/manual/5.1/manual.html#pdf-os.time">os.time</a> . Methods are provided to get and set the various parts of the date.</p>

<pre>
 &gt; d = Date {year = <span class="number">2011</span>, month = <span class="number">3</span>, day = <span class="number">2</span> }
 &gt; = d
 <span class="number">2011</span>-<span class="number">03</span>-<span class="number">02</span> <span class="number">12</span>:<span class="number">00</span>:<span class="number">00</span>
 &gt; = d:month(),d:year(),d:day()
 <span class="number">3</span>    <span class="number">2011</span>    <span class="number">2</span>
 &gt; d:month(<span class="number">4</span>)
 &gt; = d
 <span class="number">2011</span>-<span class="number">04</span>-<span class="number">02</span> <span class="number">12</span>:<span class="number">00</span>:<span class="number">00</span>
 &gt; d:add {day=<span class="number">1</span>}
 &gt; = d
 <span class="number">2011</span>-<span class="number">04</span>-<span class="number">03</span> <span class="number">12</span>:<span class="number">00</span>:<span class="number">00</span>
</pre>


<p><code>add</code> takes a table containing one of the date table fields.</p>

<pre>
 &gt; = d:weekday_name()
 Sun
 &gt; = d:last_day()
 <span class="number">2011</span>-<span class="number">04</span>-<span class="number">30</span> <span class="number">12</span>:<span class="number">00</span>:<span class="number">00</span>
 &gt; = d:month_name(<span class="keyword">true</span>)
 April
</pre>


<p>There is a default conversion to text for date objects, but <code>Date.Format</code> gives you full control of the format for both parsing and displaying dates:</p>

<pre>
 &gt; iso = Date.Format <span class="string">'yyyy-mm-dd'</span>
 &gt; d = iso:parse <span class="string">'2010-04-10'</span>
 &gt; amer = Date.Format <span class="string">'mm/dd/yyyy'</span>
 &gt; = amer:<span class="global">tostring</span>(d)
 <span class="number">04</span>/<span class="number">10</span>/<span class="number">2010</span>
</pre>


<p>With the 0.9.7 relase, the <a href="../modules/pl.Date.html#">Date</a>  constructor has become more flexible. You may omit any of the &lsquo;year&rsquo;, &lsquo;month&rsquo; or &lsquo;day&rsquo; fields:</p>

<pre>
 &gt; = Date { year = <span class="number">2008</span> }
 <span class="number">2008</span>-<span class="number">01</span>-<span class="number">01</span> <span class="number">12</span>:<span class="number">00</span>:<span class="number">00</span>
 &gt; = Date { month = <span class="number">3</span> }
 <span class="number">2011</span>-<span class="number">03</span>-<span class="number">01</span> <span class="number">12</span>:<span class="number">00</span>:<span class="number">00</span>
 &gt; = Date { day = <span class="number">20</span> }
 <span class="number">2011</span>-<span class="number">10</span>-<span class="number">20</span> <span class="number">12</span>:<span class="number">00</span>:<span class="number">00</span>
 &gt; = Date { hour = <span class="number">14</span>, min = <span class="number">30</span> }
 <span class="number">2011</span>-<span class="number">10</span>-<span class="number">13</span> <span class="number">14</span>:<span class="number">30</span>:<span class="number">00</span>
</pre>


<p>If &lsquo;year&rsquo; is omitted, then the current year is assumed, and likewise for &lsquo;month&rsquo;.</p>

<p>To set the time on such a partial date, you can use the fact that the &lsquo;setter&rsquo; methods return the date object and so you can &lsquo;chain&rsquo; these methods.</p>

<pre>
 &gt; d = Date { day = <span class="number">03</span> }
 &gt; = d:hour(<span class="number">18</span>):min(<span class="number">30</span>)
 <span class="number">2011</span>-<span class="number">10</span>-<span class="number">03</span> <span class="number">18</span>:<span class="number">30</span>:<span class="number">00</span>
</pre>


<p>Finally, <a href="../modules/pl.Date.html#">Date</a>  also now accepts positional arguments:</p>

<pre>
 &gt; = Date(<span class="number">2011</span>,<span class="number">10</span>,<span class="number">3</span>)
 <span class="number">2011</span>-<span class="number">10</span>-<span class="number">03</span> <span class="number">12</span>:<span class="number">00</span>:<span class="number">00</span>
 &gt; = Date(<span class="number">2011</span>,<span class="number">10</span>,<span class="number">3</span>,<span class="number">18</span>,<span class="number">30</span>,<span class="number">23</span>)
 <span class="number">2011</span>-<span class="number">10</span>-<span class="number">03</span> <span class="number">18</span>:<span class="number">30</span>:<span class="number">23</span>
</pre>


<p><code>Date.format</code> has been extended. If you construct an instance without a pattern, then it will try to match against a set of known formats. This is useful for human-input dates since keeping to a strict format is not one of the strong points of users. It assumes that there will be a date, and then a date.</p>

<pre>
 &gt; df = Date.Format()
 &gt; = df:parse <span class="string">'5.30pm'</span>
 <span class="number">2011</span>-<span class="number">10</span>-<span class="number">13</span> <span class="number">17</span>:<span class="number">30</span>:<span class="number">00</span>
 &gt; = df:parse <span class="string">'1730'</span>
 <span class="keyword">nil</span>     day out of range: <span class="number">1730</span> is <span class="keyword">not</span> between <span class="number">1</span> <span class="keyword">and</span> <span class="number">31</span>
 &gt; = df:parse <span class="string">'17.30'</span>
 <span class="number">2011</span>-<span class="number">10</span>-<span class="number">13</span> <span class="number">17</span>:<span class="number">30</span>:<span class="number">00</span>
 &gt; = df:parse <span class="string">'mar'</span>
 <span class="number">2011</span>-<span class="number">03</span>-<span class="number">01</span> <span class="number">12</span>:<span class="number">00</span>:<span class="number">00</span>
 &gt; = df:parse <span class="string">'3 March'</span>
 <span class="number">2011</span>-<span class="number">03</span>-<span class="number">03</span> <span class="number">12</span>:<span class="number">00</span>:<span class="number">00</span>
 &gt; = df:parse <span class="string">'15 March'</span>
 <span class="number">2011</span>-<span class="number">03</span>-<span class="number">15</span> <span class="number">12</span>:<span class="number">00</span>:<span class="number">00</span>
 &gt; = df:parse <span class="string">'15 March 2008'</span>
 <span class="number">2008</span>-<span class="number">03</span>-<span class="number">15</span> <span class="number">12</span>:<span class="number">00</span>:<span class="number">00</span>
 &gt; = df:parse <span class="string">'15 March 2008 1.30pm'</span>
 <span class="number">2008</span>-<span class="number">03</span>-<span class="number">15</span> <span class="number">13</span>:<span class="number">30</span>:<span class="number">00</span>
 &gt; = df:parse <span class="string">'2008-10-03 15:30:23'</span>
 <span class="number">2008</span>-<span class="number">10</span>-<span class="number">03</span> <span class="number">15</span>:<span class="number">30</span>:<span class="number">23</span>
</pre>


<p>ISO date format is of course a good idea if you need to deal with users from different countries. Here is the default behaviour for &lsquo;short&rsquo; dates:</p>

<pre>
 &gt; = df:parse <span class="string">'24/02/12'</span>
 <span class="number">2012</span>-<span class="number">02</span>-<span class="number">24</span> <span class="number">12</span>:<span class="number">00</span>:<span class="number">00</span>
</pre>


<p>That&rsquo;s not what Americans expect! It&rsquo;s tricky to work out in a cross-platform way exactly what the expected format is, so there is an explicit flag:</p>

<pre>
 &gt; df:US_order(<span class="keyword">true</span>)
 &gt; = df:parse <span class="string">'9/11/01'</span>
 <span class="number">2001</span>-<span class="number">11</span>-<span class="number">09</span> <span class="number">12</span>:<span class="number">00</span>:<span class="number">00</span>
</pre>



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