If you need to get back dated running numbers, you can do the following.
E.g.
Current date is Jan 2020.
I need to enter documents in Dec 2019.
This is my current running number set.
Notice the "[YY]"? This is the year portion; currently in the example row, it displays as "20"
Step 1. Duplicate the running number you want to change.
Step 2. Give it a meaningful name
step 3. Change the "[YY]" to the 2 digit year you want. In this example, I changed it to "19"
You should always verify your format setting by looking the example row.
Step 4. When you create a new invoice, the system will prompt which running number to select.
If the wrong running number was selected, you still can change it before saving the entry.
You can click on the "running #" button and choose another running number set.
The running number set name will be displayed until the document is saved. At which point, it will display the document number.
Step 5.
Once the backdated documents have been entered into the system, the old running number set can be deleted.
It is not necessary to keep the old running number set.