Excel date formula 2000 vs. 1900 conditional

I have a database of about 7000 dates, most of which are 19xx, so when I converted to the 4 digit year, it asked me if I wanted to convert it to 1900 or 2000 and I chose 1900. Well, I still have some 2000s that I need to take care of. They are noted by the adjacent column which let me know it is 2000 instead of 1900. So, what I'd like to do is a formula or something that says if the adjacent column is 20**, then this column is 20** or something like that.

I've attached a document. So basically, column B is the "real" year, but column A has the full date.

So =IF(b2=20**, a2=**/**/20**) or something like that. If b2 is not 20**, then no change.