I agree with Mark as this can keep the units consistent as well, although, displaying smart units can be nice too depending on the use case and actual time windows.
Often times we'll store data in a datetime field so it's available if we need more granular timeframes, but have "helper formula fields" to cast the datetime to a date. This can reduce keeps noise on reports by omitting timestamps when necessary and can make your formulas a bit easier to read.
For example, the field "Start Timestamp" and a helper "Start Timestamp To Date" that is:
ToDate([Start Timestamp]). Then, use consistent field types in your duration calculations.
[QC Timestamp To Date] - [Start Timestamp To Date]
And as Mark pointed out you could represent these Durations as Numeric Days instead using ToDays() and changing the formula field type to numeric.
------------------------------
Brian
------------------------------