QB Issue Resolution:

I would reconcile what is there to the year end balance.   You can do the math to get you there. Then move forward from there.    But either date would work.  If it is easier to work from 12/17.

Resolution for Issue 'What amount should I be using as the Opening Balance for an AMEX credit card?' available: Yes (Solved).
Source: Intuit Community forum.

What amount should I be using as the Opening Balance for an AMEX credit card?: 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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