Hi Susan
When you say day end, I assume you mean period end.
Transactions with future dates are a problem in Adagio, for the exact reason you explain. There is no way to selective delete transactions (or move them to history); the only way to delete transactions is based on the date.
Yes, running a day end as of 2020 would "fix" the problem, but it would move all paid transactions to history for all customers, so this is not a good solution.
The best workaround I can suggest, until Softrak adds a feature to run period end for selected customers, is to temporarily change the setting "Clear transaction detail by" in Edit Company Profile | System Options from Period End to Billing Cycle. Then create a dummy billing cycle and temporarily assign this customer to that cycle. Then, using a 2021 date, run a statement for this billing cycle, with "Select customers based on balance" unchecked. After it prints, say Yes to "Did the statements print correctly?", and say Yes to "Clear all paid transactions". Finally, change the Profile setting back to Period End, reset the billing cycle for the customer, and delete the dummy billing cycle.
This will move the transactions for this customer to history. Honestly, I'm not sure whether it will purge historical transactions - maybe someone else can confirm this, or you can test it out and see.
In any case, test this on a copy of your data first.
Steve