Time formula

I am using a formula for time calculations. Using A1,B1, & C1 where the sum is in C1. However when I enter a time in A1 it gives me a time that is different until I enter the time in the second cell then will correct itself.

The question I have is there a way to have the C1 remain blank until both times are entered into the cells to give the sum of the times?

Here is the formula that I am using,

=IF(24*(IF(A1>B1,B1+1-A1,B1-A1)),24*(IF(A1>B1,B1+1-A1,B1-A1)),"")

Thank you for any help

Try

=IF(COUNT(A1,B1)=2,MOD(B1-A1,1)*24,"")