So you have miliseconds. To get the number of minutes you divide miliseconds by 60000. To get the number of seconds, you divide miliseconds by 1000, but as seconds go up to 60, you need to take the modulo of that. To get the remaining miliseconds, just take the modulus of the miliseconds ammount and 1000. In summary:
# assuming t has the miliseconds measured value
var minutes = int(t / 60 / 1000)
var seconds = int(t / 1000) % 60
var miliseconds = int(t) % 1000
var time = ("%02d" % minutes) + (":%02d" % seconds) + (":%03d" % miliseconds)
time is de formatted string. For example in "%02d", 02 means quantity of digits, and d means integer, the % minutes after that means "use minutes number with that format".
Maybe there are better ways.. i just use that.
Also, miliseconds update to fast for you to being able to keep up showing smooth time.. perhaps you should use just minutes:seconds