I am not 100% sure I understand all the criteria...

Like what if year is greater than that in G12...

but try this and let me know what's wrong about it, if anything:

=IF(RIGHT(E22,4)+0<YEAR($G$12),12,IF(MONTH($G$12)>=4,12,($G$12-DATE(YEAR($G$12)-1,4,1))/30.3333))