Time subtraction

[Dgwood90]

I need to have a formula that will show how much battery run time I left.
A1 is Battery run time = 40.5 hours
B1 is time batteries started = 12:55 AM
C1 is time on batteries, =NOW()-B2
D2 will be the battery run time left.

I have tried formatting in several different modes, always coming up wrong.

 

[Bob Phillips]

Try C1: =MOD(NOW(),1)-B1

you don't need D2 then

 

[Dgwood90]

=A1/24-C1
For C1 to give you the time on batteries, B2 will need to be a date and
time, so that you can subtract it from NOW()