It’s nice because people intuit and talk about days as a contiguous daylight period followed by a contiguous nighttime period, when they aren’t having discussions about timekeeping. It’d be nice if we picked an arbitrary delineation that aligned to that.

Wrt doing math, realistically you could just pick an arbitrary day of the year as your reference and every day begins at a multiple of 24 hours from that. No math is needed.
I’m not looking for perfect alignment with local dawn in every location on the planet, I’m aiming for a calendar day that aligns with our intuition of the day/night cycle.
But more than that, I’m having a silly discussion about timekeeping.