Adding A Time Offset Based On Time Of Day

I am replicating a Matlab program which calculates tide levels at different times of day. I need to replicate it in excel to speed up data analysis and I am nearly there.

what happens is I need to apply a time offset to the time of high tide at port a based on the time of day, so if it is:

between 00:00 and 06:00 the high tide at port b is 81.6 minutes after the peak at port a
between 06:00 and 12:00 the high tide at port b is 74.56 minutes after the peak at port a
between 12:00 and 18:00 the high tide at port b is 81.75 minutes after the peak at port a
between 06:00 and 12:00 the high tide at port b is 79minutes after the peak at port a

I tried this formula, where CO2 has the time/date of the high tide at port a:
=IF(CO2<0.75,IF(CO2<0.5,IF(CO2<0.25,CL2+(81.6/(24*60)),CL2+(74.56/24*60)),CL2+(81.75/(24*60))),CL2+(79/(24*60)))

The problem is the high tide on 07/01/2005 07:45 is read as 38359.32 rather than 0.32 - is there any easy way to tell excel I'm only interested in the time not the date? I have this spreadsheet setup now to do all the other bits required and i is just the timing that is a problem.