To convert hh.mm in a1 to an Excel time, use:

=time(int(a1),mod(a1,1)*100,0)

If you want to enter a time in a2 with an assumed pm, use:

=time(int(a2)+12,mod(a2,1)*100,0)

Your requested formula would then be:

=a2-a1-time(0,30,0)

Regards,
Fred.

"adhide" wrote in message
...
when working out a time sheet for example, i want to be able to enter time
using a decimal point (.) instead of a colan(:) and be able to add them
together to get decimal hours.

ie. start 8.00 (8:00) finish 3.45 (15:45) minus .30minutes (0:30) = 7.25
(7:15)