I dont agree with your numbering system but....
Let ( x = YourField;
Date ( Middle ( x; 2; 2 ); Right ( x; 2 ); 2010 + Left ( x; 1 ) )
The number sequence can be anywhere from one digit to five digits in length.
This is concerning. Unless the number is always 5 digits, how will one know how to parse it?
Year 2010, month December and day 2? Or year 2010, month February and day 22?
UPDATE: And I agree with Mr. Vodka - that numbering system isn't a good one. If you are migrating or translating data, we understand but if you are using this yourself, you might seriously want to consider changing it. Even if external source, could they provide a better pattern for you?
Thanks for your advice. Yes, I agree, the database I've been given to convert wasn't created with an easy conversion in mind. This is what I have to work with, unfortunately - I don't believe the person obtaining the data understands logical thinking.