If you are an Accrual Basis entity, then the dates of the bills that were not also paid at that same date, means you would enter them as Bills, and later use Pay Bills to generate the Bill Payment Checks, for your banking entries.

For a Cash Basis entity, the date of the check is the date of payment, so you don't need Bills, after the fact, for catching up the bookkeeping.

The same is true for Sales: Invoice (accrual) or Sales Receipt (cash).

Doing that work now, is no different than is you had done it then. It's still just Data Entry, always putting in the date of that transaction and all details that apply.

Don't reconcile any accounts currently, if the historic info you intend to enter isn't already there. The data has to exist in sequence, not matter if it was entered in sequence. So, you can work in Nov 2018 while entering Jan 2018, too.