I don't recall anywhere in NPM where it queries the F5 devices for their current time, and I'd be surprised if that was something you would find in an OID.
So given that you can't get system time from SNMP your best bet is probably to rig up script in SAM to query for system time. That would all be custom so you will be kind of on your own to figure out how to make that happen.
The below article seems to indicate that the command to see current system time on a Big IP is
date ; hwclock
Within your script you would have to get the results of that command and compare them to something like from an NTP server.
Thanks. Actually I have gone through Undp process but while creating the alerts unable to get through.