You could use the session duration (/mtimer sd) for that. This displays the time since you logged in to the server or started the singeplayer world. Just set it to {hours}:{minutes}:{seconds} because by default only hours and minutes are shown. Then configure timer display to only show {sd} 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.