Excel 2007 : Need to categorize a given date and time value in a particular time slot

Hi

I have a series of rows as date and time values in one column. These are say the incidents reported date and time. I need to bucket each one of these values in one of the available time slot. Eg - if a date and time value is 1/2/2012 06:58:01 AM and i have following time slots available: 20.30 to 12.30, 12.30 to 15, 15 to 18, 18 to 20.30. This should return me as 20.30 to 12.30.
What formula should i use.

Appreciate your help on this

Thanks - Naresh

Can you attach a sample workbook of want you are trying to do?