Why Janmashtami splits across states

Janmashtami dates can differ because traditions give different weight to ashtami, nishitha, sunrise and Rohini nakshatra.

A single ashtami can extend across parts of two civil dates. Different traditions may select different observance days from that interval.

The relevant times

Nishitha is the middle muhurta of the night. A rule that requires ashtami at nishitha can select a different date from one that considers sunrise. Rohini nakshatra and other conditions also matter in some traditions.

What a date needs to specify

A Janmashtami listing should identify the tradition and the place used to determine it. A regional or household date follows the rule of that tradition as well as the tithi at sunrise.