As you are only using two digits for your year you can only have a reference to 100 different years.
Try the following code;

IF yy<50 THEN yy=yy+2000
IF yy< 0 THEN yy=yy+1900

This will convert any number from 0 - 49 to 2000 - 2049
and 50 - 99 to 1950 - 1999
You can have a higher or lower number than 50, eg. IF yy<60 = 1960 - 1059
but your program will still only hold 100 years.

Hope this helps; [email protected]