Yes, you can!
For iPhone drivers:
To add your previous mileage, you can use the "Forgot to record a trip?" button in the menu of your app.
If you have a lot of miles to enter, and would like to enter the miles in one bulk amount--no problem! You can go to the "Reports" tab in the app, click the icon to export your Stride tax report, click "2017 report," and then complete "Step 1: Add mileage" and "Step 2: Add expenses."
For Android drivers:
Unfortunately the app doesn't yet have the ability to add in past mileage in bulk; you're currently only able to add in individual trips using the "Forgot to record a trip?" button in the menu of your app.
If adding in each trip would be too time-intensive, you have a few options:
1) Keep two mileage logs, one for your driving before Stride Drive and one for afterwards. When you file your taxes in April, you'll simply add together the mileage totals that you've tracked from each part of the year!
2) Wait a few short months until we have a bulk mileage addition feature, which will allow you to add in your total number of miles in one go.
Just be sure you are only adding in mileage and expenses for which you have proper documentation, so that you can back up the deductions!