This reply is a little late, but I wonder if this is what you're looking for:
=mod(b2-a2,1/24)

I didn't put it into a pivot table, so I don't know if it will affect your
sort in the same way, but it seems to output the difference in minutes to
column C.

"Don" wrote:

My spreadsheet has two columns with two different times, as follows:

Col A Col B
11:06 12:09

These columns are formatted as Time xx:xx:xx (although I really don't care
about seconds), 24H format. I want column C to show the difference between
column A and column B, in minutes, not hours. My problem is that when I use
a Pivot Table to summarize the sum of column C, as soon as column C 24
hours, it shows 00:10:00 (ie. 24:10:00) and then the sorting in my pivot
table gets all out of whck.

Sorry--I suspect this is simple but I can't seem to figure it out.

Thanks in advance

Don