$duration is a DateTime::Duration object.

$string is a duration represented in ISO 8601 format, for example: 'P1Y' or 'PT2H29M58.682S'. $elem is the XML::LibXML::Element being parsed. $context is the document context.

The standard way of representing durations in Microformats is as an ISO 8601 string:

 <abbr class="duration" title="P4DT4H">4 and a half days</abbr>
    

This constructor also supports a number of experimental microformat duration patterns. ISO-31 class names are supported:

 <div class="duration">
  <span claa="d">4</span> and
  <abbr title="12" class="h">a half</abbr> days.
 </div>
    

As are metric/SI measures (in seconds):

 <span class="duration">124 s</span>
 <span class="duration">124</span> seconds
    

Or using an hMeasure microformat with no 'item' property, the 'type' property either absent or a case-insensitive match of 'duration' and a unit property of 's'/'sec'/'seconds', 'min'/'minutes', 'h'/'hours' or 'd'/'days'. For example:

 <span class="duration hmeasure">
  <b class="unit">Days</b>: <span class="num">4.5</span>
 </span>
    

Note that there is not always a consistent answer when comparing durations. 30 days is longer than a month in February, but shorter than a month in January. Durations are compared as if they were applied to the current datetime (i.e. now).

This function is not exported by default.

Can also be used as a method:

 $a->compare($b);
    

This function is not exported by default.

Can also be used as a method:

 $c = $a->add($b);
    

This function is not exported by default.

Can also be used as a method:

 $c = $a->subtract($b);