Invalid format output from ps etime

We have some scripts that run to detect long running processes. It works using ps and etime

e.g

ps -eo uid,pid,etime

This part works fine except that occasionally we get an etime back with an invalid format

2000 20453 1158050441-07:00:15 sh

All of the processes this happens to are short lived so I haven't caught one to investigate.

I'm very interested to why the format is coming out like this and why that epoch.

In case it helps, its a Debian Linux system running a 2.6 kernel.