529 rollover to Roth

With Secure Act 2.0, a 529 can be rolled over (within limits) if it has been opened for at least 15 years.

 

I have a 529 with my son as beneficiary. When I withdrew all the funds to pay for his college, the plan administrator closed the account. If I deposit new money, the account re-opens. But when does the 15 year start? The original open date or the re-open date?

 

Thanks much!