What should I use for date placed in service? vehicle expense?

I know the official answer is when I started using the vehicle for business and as is stated here: https://ttlc.intuit.com/community/tax-credits-deductions/discussion/what-does-date-placed-in-service...

 

However, I did not know I was using it for business or did not know I could deduct it so I never kept any records.

 

So again what date should I use. A random date when I may have started using it long ago, or the more recent date when I started keeping mileage and expense records as a proper business expense.

 

The second one makes more sense to me, but I would like know the proper way to do it. Thank you.