Working out times over midnight with a twist

We have A1 (Requested time) B1 (Allocated) time C1 (Started time) D1 (Completed time)

I am looking to report on two things (time taken to allocate a job, relative to the requested time) & Time taken from start to complete of the job.

I am using =MOD(B1-A3,1) so that any jobs requested/started before midnight and allocated/completed after midnight still give a value.

However we sometimes allocate a job before the requested time (usually just a few minutes).

Is there any way i can work my formula to provide a zero value if the requested time after the allocated time?