Hi there Alexander,
a truly good and easy solution to this issue probably doesn't exist.

I figured out one way of doing this. Assign a user defined format for the relevant cells. In this you enter the format code: "20"00-00-00 (assuming dates are always starting with 20...

If you enter 091203, the cell will show 2009-12-03, but... the cell will contain the value 91203 and you can not use it for date calculations, it actually just looks like a date.

Another, more difficult solution (but instead with true date format), would be to use VBA. Here is a description of how someone else solved it: http://www.cpearson.com/excel/DateTimeEntry.htm

No solution is really nice. I guess you how to live with some extra typing...

Best regards
Bjorn