I'm stuck trying to do some basic arithmetic using the YEAR function

I'm trying to derive the number of years that have passed since a boat was either built or underwent a major refit. I have one column for date built and another for date of last refit. First, I wanted to return a blank if there is no last refit date, and then to calculate the number of years since the last refit if there is a date in the last refit column. Once that step has been accomplished, I want to do the same for the year built column.

So far, this unparseable formula is what I've come up with:

=IF([Last Refit]@row="","", SUM(YEAR(TODAY()-YEAR([Last Refit]@row))), IF([Year Built]@row"","", SUM(YEAR(TODAY()-YEAR([Year Built]@row)))))

I welcome your suggestions!