Can anyone help me with a small problem I have with Date Fomatting?

I have an Excel based application where people need type the name of a month
into a cell, the only problem is that the cell isn't big enough to show the
entire month name for the longer months. I want to try to add some sort of
formatting so that if 'December' is entered for instance the cell is
automatically truncated to 'Dec'.
I can't seem to find anywhere in the formatting menu that solves this
problem, does anyone have a solution they could offer?

Thanks

Neil