Subtract fixed time from duration field

Hello,
I’m trying to subtract a specific time (3:30 hours) from a duration field using formula that looks like this:
Difference field = IF({Ride duration}=3:30,0,({Ride duration}-3:30))
Basically, I want to subtract 3:30 hours from the duration field to know the difference between the times

What am I missing here?

Thanks in advance,
Liat :slightly_smiling_face: