One pitfall that easily messes up time calculations:
The "time unit" in spreadsheets is one day (24 hours). If you subtract 4 from a cell containing the time 13:00:00, you are really subtracting 96 hours (4 times 24), yielding your -83:00:00.
When dealing with timespans across midnight, you need to account for that in your formula, by either including dates in the calculation, or adding/subtracting (possibly a multiple of) 24 hours.
You can add 4 hours as "+1/6" (because 4 hours is one sixth of a day), or as "+TIME(4;0;0)" (the TIME() function is specifically made to convert "human time units" to the spreadsheet time data format).
I hope this helps. If you need more specific help, post again, specifying your data in more detail.
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