Time & a half calulated on real time (not after x many hours worked)

Hi,

Can't quite find what I'm looking for.
I 'm trying to calculate time and a half after a certain time of day not after x many hours worked (which seems to be everyone else's question)

ie Standard time 7.30am-11.30pm, time and a half 11.30pm - 7.30am.

Name Start End
Simon 12/04/2012 12:00 12/04/2012 18:00
David 12/04/2012 22:00 13/04/2012 03:00
etc

I've got the rates on a separate tab, so can just link to them as I need to but need a helping hand with the formula - any ideas? 'IF" and some kind of <>?
Thanks all

N