I added the wrong bank ending date and reconciled

Let me guide you on how to change the ending date after the reconciliation, mary132.

You can still change the ending date if you haven't completely completed the reconciliation.  You'll want to go back to the reconciliation page and change the Ending Balance and Ending Date. To do this, follow the steps below:

1. Go to Accounting on the left menu. 
2. Click the Reconcile tab.
3. Choose the bank account from the Account drop-down list.
4. Tap the Resume reconciling button.
5. Tick the Edit Info button located in the upper right-hand corner.
6. Change the Ending Balance and Ending Date.
7. Press Save.

But if you have already completed the reconciliation, changing the date is unavailable. You'll want to undo the reconciled transactions instead. Please see the detailed steps provided:

1. Click Accounting at the left panel and choose Chart of Accounts.
2. Locate the account for the transaction and click View register.
3. Look for the transaction that you want to undo reconciliation.
4. In the check column, repeatedly click the top line of the transaction until the box is blank. This removes the transaction from the reconciliation. 
5. Click the Save button.

Please check this article for more information: Undo or Remove Transactions from Reconciliation in QuickBooks Online. In addition, QuickBooks Online generates a reconciliation report automatically. You'll want to print or export the report in case you want a copy for future reference: How Do I View, Print, or Export a Reconciliation Report?

Fill me in if you have any concerns about your ending date. I'll be here to help you always.