I linked my checking account in QB with a start date of 1/1/20. How do I go back and add/link some earlier transactions from 12/2019?

Welcome to the Community, @usereastsideupc-offi. Glad you've got your checking account linked with QuickBooks. QBO automatically downloads the last 90 days or more of transactions from your bank. 

I'd be delighted to help you add some earlier transactions manually from December 2019. Here's how: 

1. Sign in to your bank's website, and ensure to select December 1- 31 in the date range. 
2. Follow your bank’s instructions for how to download transactions to your computer.
3. Save the file somewhere you can easily find it in a CSV (comma-separated values) format. 

Once done, go back to QuickBooks to upload these transactions. Here's how:

1. In the Banking menu, select the Banking tab.
2. Select the blue tile for the account.
3. Select the Update drop-down menu, then select File upload.
4. Select Browse, then select the file you've saved, then click Next. 
5. Follow the on-screen instructions to match the columns on the file with the banking fields in QuickBooks, then click Next.

Once done, select Let's go. Your December transactions are now added to your checking account. You're now ready to match and categorize the transactions to make sure they're added in QuickBooks. 

For future reference, I'm also adding an article: Fix bank upload errors in QuickBooks Online. It'll guide you in case you'll encounter issues when uploading bank transactions into QBO. 

Feel free to visit again should you have additional concerns. We're always delighted to assist.