I think that it is assumed that everyone synchronises their watches/clocks with ATS at the start of each flight. It is not for ATS to worry about how accurate your time piece is or indeed to account for it.
The position one passes in a TRPACER call is the current one, ie at the instant of the call therefore no time required.
During PTLPT (ie IFR/Airways) calls, I'm guessing that the time is required over the previous waypoint since the speed and level of radio traffic might mean a considerable distance is covered between passing the point and getting the message to ATC. I guess it will also give ATC an indication of how fast you are travelling.