From http://www.w3schools.com (Copyright Refsnes Data)

SMIL Timing

Timing is about when to start and when to stop.

Timelines and Timing

Most SMIL element have timing attributes to define the timeline of the presentation.

Timing attributes defines the start time and the duration of an element.

The following table lists the possible time formats:

Format	Examples
hh:mm:ss.f	1:50:00 (One hour and fifty minutes) 10:50 (Ten minutes and fifty seconds) 10.5 (Ten and a half second)
number [h\|min\|s\|ms]	3.5h (Three and a half hour) 3.5min (Three and a half minute) 3.5sec (Three and a half second) 35ms (Thirty-five milliseconds)
wallclock (YYY-MM-DDThh:mm:ss+zone)	wallclock(2003-08-01T12:10:30+1.00) (Ten minutes and thirty seconds past twelve, August the first 2003, coordinated universal time plus one hour)

The value "indefinite" can also be used to define never ending loops.

The duration (dur="5s") attribute of an element defines how long the element will be visible:

<html>
<head>
  <style>.t {behavior: url(#default#time2)}</style>
</head>
<body>

<img class="t" src="image1.jpg" dur="5s" />

</body>
</html>

The begin (begin="2s") defines when the element will be visible (start playing):

<html>
<head>
  <style>.t {behavior: url(#default#time2)}</style>
</head>
<body>

<img class="t" src="image1.jpg" begin="2s" />

</body>
</html>