Easy:
Enter hours and minutes as 6375:25 (NOT 6,375:25 or any other way)
Assuming A1 is ON AIRCRAFT (= Installed), and B1 OFF AIRCRAFT (= Removed), then: in C1: =B1-A1 (or whatever order you decide for the columns). This will give you the answer in Hr:Min
Should you need the answer in Hrs & Decimals, then in D1 =HOUR(C1)+(VALUE(MINUTE(C1))/6)/10
Of course as the value (hours) you will insert may exceed 24:00, do not forget to format the cells in columns A, B & C (NOT D) as [h]:mm so excel will recognise the hours values instead of going into other figures such as days ...
Should you need further info, do not hesitate.
Have fun
MP