Yes, you can claim Delta SkyMiles for past flights in 2026, provided you follow the airline's specific "Missing Mileage" window. You can generally request credit for flights taken within the last nine months. If you were not a SkyMiles member at the time of the flight, Delta typically allows you to claim miles for flights taken up to 30 days prior to your enrollment date. To claim these miles, you must log into your account on the Delta website or Fly Delta app and provide your ticket number (a 13-digit code starting with 006). It usually takes about five to ten business days for the miles to reflect in your account. This is a vital practice for travelers because even a single long-haul flight can earn enough miles for a domestic "Basic Economy" award seat. Note that you must have actually flown the segment; "no-show" tickets or cancelled flights do not earn miles, and certain deeply discounted "bulk" or "mistake" fares may be ineligible for mileage accrual depending on the fare class purchased.