We have engaged a 3rd party software company to help us but the general concept they indicated was to pick a "Date 0" if you didn't have solid information to work from. You're early enough that if you start data aggregation now, you should be in good shape having a decent amount of data by the go-live date.
Born once? Die twice.
Born twice? DIE ONCE!
Loan Review, HMDA, ALLL
Opinions are my own and do not reflect any others