Excel Time Calculation

I have an excel sheet where i want to calculate excess time.Column 1=date,Column 2=Time in,Column 3=Normal Time in,Column 4=Excess.

Conditions:
Normal Time in =8:45
If a user clocks in before 8:15 then excess =8:45 minus 8:15 =30 mins
If a user clocks in after 8:15 then excess =8:45-time in
if time in >8:45 excess=0