“Would I have to go back through all of the bank statements and reconcile, or is there an easier way?”

You have to pick a point and start there.

“and have nothing to do with filing his taxes.”

But that would work as a Reference. You need to Start somewhere.

For example, if 2015 […]

Resolution for '2015 data issue: 'I was wondering if someone could help me with a new client? The client is an S-Corporation that has never reconciled their books with their bank statement before.' available: Yes.

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