First I recommend to not fall into ta trap of trying to use how you might be displaying items in the report in a header/detail/footer as the way to get to totals. Display does not equate to actual data. What I mean is, just because you can get your report to look like a simple math equation does not mean that you altered the actual row data which is what is used for the actual calculations. Meaning, the "Work requested on" value(s) is still in every row of data even though you are only displaying it in the group header.
So if you use it in a row level formula it is calculating only based on the information in the row, not the group header value. So it is not doing calculations based on how you would do it in you mind by looking at the information displayed.
Don't get me wrong, the display is important to calulations in as much as setting up groups, sort orders, using next()/previous(), etc. but making it look like a simplified math problem does not actually make it that way.
With that said, I still do not understand what you want... Do you want the minutes difference between '04/15/15 01:34:55'' and "04/16/15 14:57:12" (e.g. ticket requesed date to last service completed date? or something else?
If it is something else, what is that something else based on all of the data available, not necessarily just what you are displaying.
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