We have a prostar 816plus system which has started dropping calls on certain lines and the date and time continually change.

I have turned the system off and reinitialized the RAM, etc. but it has not corrected the problem. Our phones lines are via a T1 line and our phone company checked the T1 router and it has no errors or problems. Does anyone know what could be causing this and a solution?

Your good old Prostar is giving up the ghost.
Solution: start looking for a replacement (not on ebay those are the same as yours) ship it to a quality repair shop or look at something new.

also check your ground

You said Telco checked the router! Has Telco physically come out to see if the lines are really in specs? But you said this happened on certain lines. Have you tried to move the lines around to see if the problem follows the line or the port on the ksu? About the time issue, Do you exit program mode using mmc 20 or let it time out? I usually do this to exit program mode seems that when I don't certain 816's grey or black the date and time go crazy thats the reason I ask. To exit #20****(passcode) then 4 #. The 4 at the end keeps 12 hour clock format. The 816's are getting old and outdated, But They were pretty good systems for the time. I have a lot of 816 experience doing at least couple hundred installs!