Why does my formula change to a number?
 

I have a formula in a cell that adds the start date/time of an event to how
long the event lasts. However, when I type the formula into the cell, it
automatically calculates the end date/time, and replaces the formula with the
value of it.

How do I keep my formula in the cell?

Ex: A1: 02/23/05 12:00 pm
A2: 0.5 Hrs
A3: =A1 + (A2/24)

When I click 'Enter', it gives A3 = 02/23/05 12:30 pm , the correct result,
but it automatically replaces the formula in the cell with its value.