You can't without code. (Probably a worksheet_change() event), you can use a
helper column though if the numbers are all 8 digits long (It can still be
done if they are shorter but we will need to accont for this. Let's say the
first number is in A1, in B1 enter
Assuming (1) the leading zero is really present, and (2) use are using US
date format (mm/dd/yy)
then use =DATE(2000+RIGHT(A1,2),LEFT(A1,2),MID(A1,4,2))
Pleases note there is no difference between a date and a serial number other
than the applied format.