calculating time using numbers

I want to calculate time taken on a task. The data are: start time, end time, and time in between taken for rest or non-task use. I put down these numbers in cells formatted for time but not date:

B3: start time: 12:30AM

B4: total time off: 0:15

B5: end tine: 8:30 AM

The formula= B:5-B:4-B:3 yielded an error message to the effect that times cannot be subtracted from numbers. As noted: all cells were formatted as time. Also, =B:5-B4 yielded an answer. =B5-B3 also yielded an answer. How do I fix this problem?

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