How can I manage expenses and have them appear on QB like keeping track of mileage?

When you create an expense, you have the option to assign it to a different date than the current date, @bfprotectionserv.

To create an expense transaction separated by date, just record the date separately. If you already paid a business expense and need to record it in QuickBooks, here's how:

1. At the upper left corner, click on +New and select Expense.

2. Select the vendor in the Payee field.

3. Choose the account and enter the date of the expense in the Payment date field.

4. Select the Payment method and optionally enter a reference number.

5. In the Category details, enter a label in the Tags field to categorize your expense.

6. Select the expense account from the Category dropdown, enter a description, and if needed, specific products or services in the Item details section.

7. Click on Save and close.

Moreover, I've added these articles for a more detailed guide: 

Enter and manage expenses in QuickBooks Online

Automatically track mileage in QuickBooks Online

If you have more concerns, feel free to reply to this post. I'm always available. Have a good one!