Try this:
Assuming column A are the dates (ie. A1 to A123), and column B you flight/block times (B1 to B123), then in the cell you selected insert the following furmulae:
=SUMIF(A1:A123),">="&TODAY()-90,B1:B123).
OR
There is an other possibility based on NOW instead of TODAY, but this needs one extra column and goes as follows:
Assuming column A are the dates and column B the Blocks ON times, then column C will contain the sum of A + B in their respective rows (and formatted as dd/mmm/yyyy hh:mm) - this column could be hidden later on as it is only for calculation purpose. Column D would contain your flight/block times.
The formulae would then be:
=SUMIF(C1:Cxx),">="&NOW()-90,D1: Dxx)
For you to see.
Happy contrails
MP