It would be I3 which would be empty as this would signify a "rest period".

"MBC" wrote:

I am trying to work out the "shift Gap"(Rest time) between two shifts which
are on different worksheets in the same file. Worksheet 17 cell ref AB3
contains my finish
time of 17:00 and worksheet 18 cell ref I10 contains my shift start time of
8:00. The answer is 15 but the formula I am using gives me an answer of 17?.
Can't see where I am going wrong.
=IF('Week 17'!AB3=0,24,IF('Week 17'!AB3I3,('Week
17'!AB3-I3)*24,24-(I3-'Week 17'!AB3)*24))