If it's an issue then you CAN manually add up the ingame time mid-run. That's effectively what the autosplitter in LiveSplit does for you. A couple of things to note if you're going to do this:
The ingame time is the sum of all of the level times, not the time displayed on your save file. That is a different time value, and does not correspond to IGT.
When adding up your level times, all deaths/restarts must be accounted for, and this is typically why people prefer to use LiveSplit as it automates the process. It's still easy enough to calculate this yourself with a page like http://www.unitarium.com/time-calculator as long as you're adding the correct values. Things you should be aware of including:
Restarts count toward your IGT. If you mess up 5 seconds into a stage and restart, then finish the stage in 1:30.00, then your IGT for that stage is 1:35.00.
Deaths also count towards IGT, and this is a little trickier to calculate. If you haven't hit a checkpoint, this is treated as a restart and you just do as above, add the time when you died to your finishing time. If you died and respawn at a checkpoint/after collecting an emerald shard, then you must ADD the time of death, and then SUBTRACT the time of respawn. This will add the difference in time between when you touched the checkpoint and when you died. From there, add final level times as normal.
