The basic calclulation would be:
GetAsDate ( Timestamp2 ) - GetAsDate ( TimeStamp1 ) + 1
But I am guessing that the timestamps are in different records here.
If so, you'll need to explain the design of your table and report layout a bit more before I can be sure that what I suggest works in your specific situation.
Summary fields that return the minimum and maximum timestamps can probably be used here, but you may need to use GetSummary if your report is a summary report grouped with sub summary layout parts and you need this interval calculated for each sub group of records.