Adding onto this twins born near midnight around 1999 to 2000 could be born in different milleniums.
At least for German law (and probably other ones) that's what de facto would be required: You enter the exact town the child was born in, if known (but when moving 800 km/h over invisible town boundaries, who takes note of in which town you were at the exact moments the two were born?), or the place where the mother sets foot to ground otherwise.
Adding onto this twins born near midnight around 1999 to 2000 could be born in different milleniums.
Actually, this millennium began 2001.
Wow I learned something new. I had to search it after reading your comment and it is indeed 2001. https://www.timeanddate.com/counters/mil2000.html
Yeah, most people get it confused with the Willenium which did end in 1999.
Nice I did not know the fresh prince made an album called Willenium.
And twins born during DST changeover could have the same birth time down to the second, but one hour apart.