I am trying to calendar required filings on 1 Sheet, if possible. Thank you in advance for sharing your wisdom!

Filings are calculated from the First Date. Some filings have annual First Date anniversary Due Dates and others, problematically, are due every other year (Biennially). I would like the calculation to show the next Due Date and automatically update year to year. As another wrinkle, filing window opens 5 months before Due Date, we use this date as the trigger so it will be helpful to calculate this as well.

Sheet formatted as table, components as follows (no committed to column location of reorganization will make clear):

$A2= Annual/Biennial Filing Requirement;
$D2= First Date (base date for calculation);
$E2= Filing Window opens - 5 month's before filing date, currently using the following (suppose this could be Filing Due Date - 5 months, but I lack knowledge of this as well):
=DATE(YEAR(TODAY()),MONTH($D2)-5,DAY($D2))
$G2= Filing Due Date, currently using the following
=DATE(YEAR(TODAY()),MONTH($D2),DAY($D2))
This works great for the Annual Filings, but not so for Biennial.

I am thinking it will involve something like: 

=DATE(If($A2="Biennial, [YEAR(TODAY())PLUS Some formula],YEAR(TODAY())),MONTH($D2),DAY($D2))