Calling Private Sub/Function

So how can you call a Private Sub or Function? Or is that simply not possible? I have an OnTime event that needs to call another Sub when the timer runs out, but that other Sub is private.

Edited to add: I meant calling from a different module or userform. I know I can call from within the same module, but this is a Userform trying to call a Private Sub... Any way of making that work?