Are you sure this is correct? Technically midnight is on the next day (i.e., 11:59pm Sunday night is on Sunday, the next minute it's Monday), but the policy says you have until "midnight the day after the eTicket is purchased", and in that context Delta is clearly thinking of midnight being on the previous day. For example, if I book a ticket on Sunday for travel on a much later date, I have until 11:59pm Monday night plus one minute to cancel -- so technically midnight Tuesday, two days later.

I don't think I've ever tested this but I've always read the policy to say that if you book a ticket the same day as departure, you have until the end of that day to cancel, meaning you could risk-free cancel after the flight departs. So if you book for the next day you have the same risk-free cancel window as any other longer-term booking.