Formula to find missing dates for invoices.

[Kyletok]

• I have an invoice table with all the invoices for all the accounts.
• Each invoice belongs to an account.
• We log the Invoice Date for each invoice.
• Each account should have 1 invoice per month.

I want to make another table where all the account numbers are listed, and the second column would be "Missing Invoice", which would pull the missing date (aka could not find an invoice for the month of), and if there isn't any missing invoices it'll just say NA.

so, if Account 001 has an Invoice for January, and an Invoice for March, but no Invoice for February, then the table would have February in the second column, letting me know there's an invoice missing.

It would compare with the earliest date for that account and the latest, and find the dates missing in between. So if account 002 has January and February, but not march, there's no need for it to show march because nothing has been skipped yet.

also, the invoice date is exactly the same every month, so if it were 1/20/19, itll then be 2/20/19 - so there's no need to show just the month if it makes it harder, and just show the date missing in the sequence of numbers.

could someone help me figure out how to pull this off? assuming itll have to be an array formula.