Check Date format before proceeding

I have quite an extensive spreadsheet set up for entering data and saving etc.

I use VBA to bring up a calendar(G1) for the date to be selected by the user.

However, i have found some users tend to manually edit the date if a job was >1 day, so they enter say [2 & 3 /1/2013].

This then causes other VBA errors due to the date not being a valid format as dd/mmm/yyyy.

Is there a quick way to use VBA to confirm that a valid date format in dd/mmm/yyyy has been entered?
I could then run that BEFORE the data is saved with an incorrect date format.