Summary:
Total play time/kills/etc displayed in the player's 'Career' only track the last play session.

I connected yesterday for the first time since launch, created a DH and played around 4 hours in single player. I got disconnected twice, but got back in without hassle. Since the last time I reconnected, I played around 20 minutes, and quit for the day.

Before exiting Diablo, I checked my 'Career' window and it displayed 17 minutes as my total play time with the character (probably the amount of time I played since my last reconnection) rather than the total play time since the character creation (around 4 hours; all 4 hours were spent in-game, not in the lobby or menu). Career kills were around 121 (when they should have been several hundreds), so it seems only the stats since the last reconnection were conserved.