Put in Cell3 00:15 as 15 mins. Then in Cell4, enter the formula =if(cell1>90,Cell2+Cell3),0) you can replace the 0 at the end to be whatever answer you want for cell1 being below 90.
As for working the out the difference between 2 times that's in AM/PM, simply do 2nd time minus 1st time to get the difference. Then in next 2 cells, use the function =hour and =minute against the cell with the time difference. This will give you the hours and minutes in number form.
That should work I think!
