l am doing some reporting on Lync - from the Call Details database (LCSCDR).

I am calculating the call duration using the SessionIdtime as the call start time and the SessionEndTime as the call end time.

I have noticed that on some occasions this gives a negative result  (ok not very often)

"SELECT  DATEDIFF(ss,sd.SessionIdTime,ISNULL(sd.SessionEndTime,sd.SessionIdTime))

FROM SessionDetails AS sd".

My questions are

a) why is the sessionendtime sometime null ?

b) why is the sessionendtime sometimes less than the sessionidtime

c) what is the responsetime ?

a) I'm not entirely sure, perhaps it wasn't recorded or the session was just lost.  i.e. there was no BYE sent.

b) It shouldn't be, one's the time of the session request, the other is the time at the end of the session.  That's odd, are all of your front end servers and pools on perfectly synchronized time? 

c) responsetime is the time of the response to the first INVITE message. 

A little more detail here, but I've found it best to create test cases within the database (by making misc calls and sessions) and review what the data looks like.

https://technet.microsoft.com/en-us/library/gg398589(v=ocs.15).aspx