What date is used when you void an old check, the current date or the date of issuance?

In my experience, you would want to use the current date. 

I'm happy to share further details about voiding old checks and guide you on how to do this in QuickBooks Online (QBO), @ejnaddy.

When you void a check, I agree with @Just_me, QuickBooks automatically uses the current date. Then, you'll still have a record of it, but it won’t affect your account balances or reports. To do this (from the Check page), here's how:

1. Go to Bookkeeping, select Transactions, and then select Expenses.
2. Select Filter.
3. In the Type field, select Check.
4. Choose the date range in which the check was received and select Apply.
5. Select the check to void from the Expense Transactions list to open it in the Check screen.
6. Select More, and select Void from the pop-up menu.
7. When prompted, select Yes to confirm you want to void the check.

Once you're done, you'll want to verify the record of the said transaction in the Audit Log page. Please see the screenshot below for your reference, or visit this link for the complete guide: Use the audit log in QuickBooks Online.

Also, to further guide you in voiding and managing transactions in QBO, I'd recommend checking out this article: Void or delete transactions in QuickBooks Online.

Let me know in the comments if you have other concerns about voiding checks and managing transactions in QBO. I'm always ready to help. Take care, and I wish you continued success, @ejnaddy.