Hi
It is unclear what you want.
the 10 hours you show is the difference you get if you subtract 6:30 AM from
4:30 PM
With 6:30 AM in A1, and 4:30 PM in B1
Time difference =B1-A1 Format>Cells>Number>Custom [h]:mm
If time is likely to go over a 24 hour period use
=MOD(B1-A1,1)
If you are staring with the time in A1, and want to add 10 hours to it to
show the result that is in B1 then either
=A1+TIME(10,0,0)
or
=A1+10/24
The latter is because Excel stores times as fractions of a day (24 hours)