How can I extract year (yyyy) from user entered date (mmddyyyy)?

Thanks for everyone's input. My client decided to go with something not as complicated but these answers definitely pointed me in the right direction.

Thanks!

Your use of the date format without item separators  could cause problems when users drop leading zeros. You should verify the length of the entered value and need to be careful to not drop the leading zeros when present.

I would convert the date string to the date object and then use the "getFullYear()" method to obtain the 4 digit year. I would also include the test to verify that the date is a valid date.

Extracting the year is easier then adding the slashes. To do the former you just need to extract the last four characters of the field's value, using the substr method, for example. You can do it like this:

this.getField("FullDate").valueAsString.substr(-4);

The latter is more complicated. You would need to write custom format and validation scripts to do it.