If you're using Skype on Linux, it's the number of seconds of the call. Skype on Linux hasn't been updated in so long that the client doesn't always know what to do with what it's sent; if you're using Skype on Mac, that's when it tells you that you just got off a call with so-and-so for such-and-such time, but on Linux, it just prints out the literal number of seconds and it's done.