Originally Posted by Often1

No, it's not. If you read DL's COC you will see that this change is not even remotely a breach of the contract. And, it's unhelpful to OP who has a legitimate question and problem.

Turning to OP's question, the best approach here if you really feel that the 15 mins. means that you have to leave the event which underlies the trip early, is to call DL and explain in very brief terms that you can't leave earlier for business reasons. Can they acommodate you by doing the following.... (Propose the specific flights you want). You will have a more likely positive reaction than through threats of breach of contract.

The worst that can happen is that DL says "no."

Some practical advice: I wouldn't spend a whole lot of time on this. If you can't get an easy and quick resolution, book a car service which can meet you at your event and whisk you to the airport. Only you know the route, but hopefully that cuts enough time from the drive-time to the airport, to make up your 15 minutes.

Good luck.