What you are doing wrong is assuming that Excel knows you mean five and a half
hours when you use the string "5:30" in the Boolean expression and not the
string "5:30".

In formulas, it is best to use unambiguous representations.

You could use

TIME(5,30,0)

or

TIMEVALUE("5:30")

or even

--"5:30"

Excel will try to convert the value to time if it is used in a mathematical
expression, so you can get away with "0:30" when you are subtracting it.  But
not so in the Boolean expression.

--ron

For you information, 5.5 hours = 1/24*5.5, half an hour = 1/24*0.5
therefore , the formula should be

=if(d11-c11>=1/24*5.5, d11-c11-1/24*0.5, d11-c11)
then set the answer in time format

or
=text(if(d11-c11>=1/24*5.5, d11-c11-1/24*0.5, d11-c11),"hh:mm")

"John Smith" <toysnivy@ptd.net> ???????:sRucnVMqav8Lec7YUSdV9g@ptd.net...