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It should be "D/S", not D15, which has no specific ending date. "D/S" means the I-94 is valid as long as you keep your F-1 status. You may check the I-94 to see whether it is D/S at this website (click "GET MOST RECENT I-94" at the website):

https://i94.cbp.dhs.gov/I94/#/home