Adding an another IF(AND) isn't working

I think that this might be what you are looking for:

Formula:

=IF(AND(MOD(ROW(),2)=0,B38<>""),(LOOKUP(B38+16,$P$4:$P$11,$P$5:$P$12)-B38)/365,IF(AND(MOD(ROW(),2)=1,B38<>""),F37-(B38-B37)/365,""))

You have mod(row(),1)=1 this doesn't return a value of 1. It will always return 0. I made the assumption that you meant IF(MOD(ROW(),2)=1 was what you meant.