The purchase agreement of our new home is signed in July. Escrow will be closed in Jan 18. Which Date can be used as "purchase" date for mortgage interest deductions ?

I am trying to figure out tax implications of our home purchase with new tax reform. Home "purchased" before 15th dec can be grandfathered for 1M limit. Can we qualify for it ? or the "purchase" date is actually the date when Escrow is closed ?