Just an idea: What about setting a variable for each lap? In this case, you know the time for each single lap and you can easily add those values up to a total time.
4 Variables are needed in this case:
Lap - Integer (to track which lap the player currently is)
Lap1 - Integer (Time in seconds of the first lap)
Lap2 - Integer (Time in seconds of the second lap)
Lap3 - Integer (Time in seconds of the third lap)
I am pretty sure that there are better solutions, but I am new with this as well and this is just something that would be easy to do :)