Confusing timestamps on voice calls

I noticed a weird inconsistency with timestamps on voice calls. After a call is finished you see information about it in the chat, e.g. "You called John Doe (15m 42s)  [timestamp]".
The problem is, that it initially logs the start-time of the call but changes to the end-time after a restart. This is really confusing and i cannot conceive of a logical reason for it to be this way. It should be either or, not both and it definitely should not change.