Firstly, the xtset command actually doesn't require you to have a time variable. You can simple xtset with id and stata allows you to work with it. All you lose is the ability to use STATA's time series functionality, which given your problem, I assume you don't want to model anyway. If you really want to use a time identifier, then @dimitriy has one way to do it, but then again I don't think it helps your analysis.Given that you know users can post repeatedly on the same day but you don't know exactly when they will post, using any time series function doesn't help.