Overtime rates

For my sins, I am developing an admin application that will include processing overtime claims. At present, there are three types of overtime:

Bank Holiday OT is paid at all times on bank holidays.
Unsocial Hours OT is paid at double time Friday, Saturday & Sunday nights from 2300 to 0300.
Standard rate, paid at X1.5, applies at all other times.

This I am assured is set in stone and will absolutely never change, so I'm expecting new types rates and/or times any day now.

Our administrators, who will have to do the data entry are very busy and of, er, variable quality, so I'd like the system to work out what rate is payable. There could be all three types in one shift - a 1900-0700 OT shift on a Sunday into a Monday bank holiday, for example, would get standard rate from 1900-2300, unsocial from 2300-2400 then BH from 0000-0700.

So far, my efforts have produced a migraine and an idea for a horrendous table with every hour from Monday 0000 to Sunday 2300 and its type, another with types and rates, and a ghastly processing overhead. Surely there must be a better way. Any ideas, please?

It'll be an SQL server/VB.net application, but I'm looking for general ideas, rather than a software specific solution.