For me its not a date that makes a record, its the location.
I think something like the angle of the sun rays seems to be more important.
For exampel 31.12 somewhere in Swiss: ~19,0° compared to 21.12 at "Neustadt an der Orla": 15.8°
That´s why for me this paper holds the Record:
DITTMANN, G. (1978): Jahreszeitlich außergewöhnliche Trächtigkeit von Kreuzotterweibchen im ostthüringischen Hügelland. – Veröff. Mus. Stadt Gera, Naturwiss. R., 6: 91-92