ticks to ms in midi file

This is making me a little nutty... (and it's kind of embarrassing)

But how do I get the absolute time, in ms, for midi events in a midi file? The basic conversion is no problem, but when there are tempo changes I get completely confused. I see that it must be a cumulative time (that is, I have to keep the absolute time of the tempo change as some sort of offset), but I can't work out a reliable way to do it. Sad, I know...
Does anybody have java code to do this? I know the seq object obviously does it, since it can export text files with absolute millisecond timestamps... Anyway, I realize it's not rocket science, but I keep going round in circles, so I though I'd ask.

thanks in advance,

J.