Yes, you can!
For iPhone users:
To add your previous mileage, you can use the "Add a trip manually" 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 menu and click "Import past mileage log." You can add in a bulk number of miles that you drove before using Stride Tax.
For Android users:
To add your previous mileage, you can use the "Forgot to record a trip?" button in the menu of your app.
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.
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 Tax 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!