The simplest way to get this is to create an action with an Ends on date and time and then export the action from the console.
This will give you an XML file.
Look for these tags
<HasEndTime>
<EndDateTimeLocalOffset>
If the fields are not obvious try creating one action with the date set and another without it set and compare.
What type of server are you running ?
On Linux the xsds are in /opt/BESServer/Reference
and on Windows are in c:\Program Files (x86)\BigFix Enterprise\BES Server\Reference