2 options here.
One is to format the cells as text. At that point what you type is what you
get. The down side is that XL will not recognize the value as a date and as
such you can not perform any functions on it such as adding or subtracting
days.
The other is to format the cell to display the date as you desire it to
look. This would be my preference. The date can be typed however you want but
the display will always be the way you want. So you could type Jan 10 2010 or
1/10/2010 or just jan 10. Format | Cells... | Number tab and select Date and
the the appropriate format.