Put in say, C1: =(B1-A1)/24

The above presumes B1 will always be =A1

Perhaps a more apt formula could be:
=IF(A1<B1,(B1-A1)/24,"")

which'll return blanks: ""
if A1=B1

--
Rgds
Max
xl 97
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GMT+8, 1° 22' N 103° 45' E
xdemechanik <atyahoo<dotcom
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