Advanced Payment Term Question

I have a client that requires invoices end of month invoice date + 85 the 7th month.

This means day 1 to 7 + 3 months   the 7th  / days 8 - 31 + 4 months the 7th  This is correct for months with 31 days.

For months with 30 days it should be day 1 to 6 + 3 months the 7th / days 7 - 30 + 4 months the 7th.

For february iit would still be different dependent whether the month is 28 days and once every four years 29 days.

I can put everything on the 30 month algoritm and then the client is happy because the invoices made on the 7th of a 31 day month receive a month extra.  The february invoices are troublesome but helas.