Hi! Welcome to the forums!
Answers are usually better if you can provide details. Like what PLC you are using (Some PLC Timers count up, others count down). 
Here’s a jython snippet that converts an arbitrary number of milliseconds to an h:m:s string:
def millis2hms(millis):
from java.lang import String
from java.util.concurrent import TimeUnit
hms = String.format("%02d:%02d:%02d", TimeUnit.MILLISECONDS.toHours(millis),
TimeUnit.MILLISECONDS.toMinutes(millis) - TimeUnit.HOURS.toMinutes(TimeUnit.MILLISECONDS.toHours(millis)),
TimeUnit.MILLISECONDS.toSeconds(millis) - TimeUnit.MINUTES.toSeconds(TimeUnit.MILLISECONDS.toMinutes(millis)))
return hms
Without further details, this is a far as I can reasonably go, but I hope this points you in the right direction.