You could use the session duration ( ) for that. This displays the time since you logged in to the server or started the singeplayer world. Just set it to because by default only hours and minutes are shown. Then configure timer display to only show and set position, size, etc. as wanted.
Since mTimer is no speedrun timer it is only exact to a second based on server ticks. You might want to use LiveSplit for exact time tracking of speedruns.