...
every day at 13:00 and 20:00
<timeStep times="13:00 20:00"/>
every hour at each half hour (0:30, 1:30, etc.)
<timeStep minutes="30"/>
every hour at 5 and 45 minutes
<timeStep minutes="05 45">
every 12th of the month
<timeStep daysOfMonth="12"/>
...