I am trying to ask that a percentage is returned whether a time
entered into cell B10 falls within one of four blocks of time. It is
working for the first three time ranges, but the last one is giving me
zero for all times entered in that range (6pm - midnight).

Here is my almost working formula:

=IF(AND(B10TIME(24,0,0),B10<=TIME(6,0,1)),
100%,IF(AND(B10TIME(6,0,1),B10<TIME(12,0,1)),
75%,IF(AND(B10=TIME(12,0,0),B10<TIME(18,0,1)),
50%,IF(AND(B10TIME(18,0,0),B10<TIME(24,0,1)),25%, 0))))

I think the problem is my 'false' entry, but I don't know what should
be there. Can you help?

Thank you!