Midi time stamps unit confusion

I'm trying to build a sequencer, but I don´t understand what time stamps are. Are they expressed in microseconds (that's the java way) or are they expressed in samples?

See http://www.juce.com/api/classMidiFile.html#ab18143cad013db21a71735b01aef2a9f


Also see: http://www.juce.com/api/classMidiMessage.html#a9a942c96a776e80e3c512058b29011a8


I saw the api but i din´t saw that part. Thanks a lot!!!!!