Delta Air Lines (DAL) should not record the entire $1,000 as revenue for 20Y6. Instead, Delta should allocate the $1,000 revenue to (1) the flight flown in December and (2) the 2,000 mileage credits. Only the portion allocated to the flight flown in December should be recorded as revenue of 20Y6.
The portion of the $1,000 allocated to the 2,000 mileage credits should be recorded as unearned revenue (liability). Only when the mileage credits are redeemed will they be recorded as revenue.
At the end of its accounting period, Delta will need to update its estimates for mileage credits (unearned revenue), including mileage credits for tickets that have been purchased for flights that are yet to be flown.