daddylonglegs,

You are correct, it's an artifact of the math: 1998 + 4 = 2002

Have not tried your formula, but this seems to work:

=IF(D3="",0,IF(E3>0,SUM((YEAR(E3)-1)-(YEAR(D3)+1)+1),SUM(YEAR(TODAY())-1)-(YEAR(D3)+1)+1))