Time Fomat in Excel 2003

Hi,

I’m looking for a custom format (or whatever else works) that would turn a four digit number into a formatted time.

For example: A user types in 1030 into cell A1 for the amount of hours put into a work day and once he/she hits the Enter key cell A1 changes to 10:30.

This is necessary to facilite Excels other time formulas for calculation. Is it possible?