Let's there's a process that begins at 1:00 and ends at 3:00.

So 1:00 belongs to hour #1. 60 minutes later it is 2:00. But what about 2:00? it belongs to hour #2.

So there are 59 minutes, 59 seconds in hour #1. The minimum time slice is a whole minute. There's no accommodation in the script for fractional minutes so those extra 59 seconds get tossed out. The same is true for a process that begins at 1:30 and ends at 2:00. There are actually 29 minutes, 29 seconds, 9999 milliseconds and so on consumed in hour 1.

If you round the 59 seconds up to a whole minute then you risk counting more minutes in a day than actually exist, as the last minute in each hour would be counted twice, once at the end of the hour, and then again at the beginning of the next hour.

The solution would be to account for fractional time slices.