I understand you, @chris75.
And I also appreciate you for going through the steps provided above. Please know this is not what we want you to experience with QuickBooks.
Just for clarification though, are you referring to the date the item is created? If so, you can still merge inventory type items even if the dates don't match as long as they are the same type and name.
However, you can't merge items if both have sub-items. You must remove the sub-items first from the items you want to merge. Let me guide you how:
If there are no sub-items on the items you want to merge. Let's create a sample company file and merge items from there. Here's how:
In your QuickBooks Desktop account.
Here's an article you can read for more detailed steps: Create a QuickBooks Desktop Company File.
Once done, let's create an inventory type item with different date created and merge them. Here's an article you can check for the detailed steps and information: Add, Edit, and Delete Items.
If your able to merge the inventory item in your sample company file, then we'll have to verify rebuild your original company file to check if there are damage data in your company file. Here's how:
You can also refer to this article for more detailed steps: Verify and Rebuild Data in QuickBooks Desktop.
Also, feel free to check this helpful article for reference in case you want to learn how to setup advanced inventory in QuickBooks Desktop.
If you have other concerns or questions, please feel free to leave a comment below. I'm always here to help.