This is the date you want to carry across from your previous system. You should aim to have the same unearned income balance as your spreadsheets. It is best practice, to have this date in the past.

For example, if you have an opening balance transaction date on 31/07/19 and activate the change on 15/08/19. You will not see your transactions if you filter for 01/08/19 to 31/08/19. You will find the transactions in the month of July.