Same Day Confirmed...and it is on the site (though not necessarily expressed as "SDC"). Below is a quote from Delta.com:


Same-day Confirmed Travel

The same-day confirmed option allows you to change your flight time on the same day of travel for a fee of $50. To use the same-day confirmed option, you must confirm your new flight within 3 hours of the scheduled departure time. If you have a flexible fare (usually a refundable ticket), you may be able to change your itinerary without a fee. The same-day confirmed program is subject to change and the rules in effect on the date of travel will apply, regardless of the date of ticket purchase.

You can use the same-day confirmed option for travel within the United States, Canada, Puerto Rico, and the U.S. Virgin Islands on Delta and Delta Connection® flights.