Welcome to the QuickBooks Community. I'm happy to hear that you've been taking advantage of the robust features that come with QuickBooks Online Payroll. It's critical that what you've entered in QuickBooks reflects what you've actually paid your employees so that the system can calculate the correct taxes and forms. Let me explain why you're having trouble making these adjustments.
In QuickBooks Online Payroll, you'll only be able to enter paper cheques dated as far as 6 months back, as you've speculated. Luckily, there's a way around this that'll allow you to have these changes entered. It'll just take a few extra steps and the help of our awesome payroll support team.
When it's convenient for you, please reach out to our support team via one of the channels below. These methods of support allow our agents to collect some sensitive data from you that's not safe to share in this public forum. Once you've explained what you're trying to accomplish and an agent has pulled up your company, we'll be able to send you an template and accompanying spreadsheet that you'll use to outline the pay edits you're looking to make. When complete, you'll send the form back to your agent who can then escalate it to our Payroll team to make the changes.
Phone: Our phone support agents can be reached at 1-855-253-1536 from Monday to Fridaybetween 9 a.m. and 8 p.m. EST.
Schedule a Callback or Chat: Click (?) Help in the upper right > type and enter "Contact support" into the QB Assistant > click Contact Us > explain your situation > click Let's Talk > choose Get a callback or Start a Chat. Support is available at the same times as listed above.