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Subtracting a duration from a clock time is easiest in minutes from midnight: 17:30 is 1050 minutes, less 225 minutes gives 825, which is 13:45. When the result goes negative it wraps into the previous day, and the day offset records that - which is what matters for overnight shift calculations.
Minutes from midnight
minutes = hours x 60 + minutes
Wrap-around
add 1440 minutes and decrement the day while the result is negative
Because base-60 borrowing is error-prone by hand. Converting to a single number, subtracting and converting back avoids the mistake.
The result wraps by adding 1440 minutes and the day offset goes to -1, so 01:00 minus three hours reads as 22:00 the day before.