QB Issue Resolution:

You can UNreconcile and do just do it again with the right date.  (Prob. what I’d do just because I like the details to be correct….)

 

However….   If it was a big month and assuming you were able to Reconcile without any errors/adjustments, and there were no transactions on the dates you were off by, then just leave it.  If the bank doesn’t show any transactions for the month you Reconciled and will not show anything in the next month on the days in question, it won’t affect your balance.

Resolution for Issue 'Can I change ending date Bank Statement on Bank reconciliation already finished' available: Yes (Solved).
Source: Intuit Community forum.

Can I change ending date Bank Statement on Bank reconciliation already finished: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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