Why does a day end when a new day begins?

Like how does a day end at 12:00am when a new day is starting? Is there a point where 12:00am is neither day of the week?like it's been 24 hours at the point of midnight the next day but you'd have to say for instance if the day started on Monday it'll send at the start of tusday... Or mone theres not been any time gone by at midnight it's just Carry's on into the next day

Bill2019-12-27T05:21:51Z

It's one second to midnight, then 1/2 second, then 1/4 second... and wait until you start thinking about time zones!

?2019-12-27T05:17:59Z

Are you high or something?