Krishna’s birth, at midnight — so nishita decides it, not sunrise.
Krishna Janmashtami is Bhadrapada Krishna Ashtami — the Ashtami of the krishna fortnight of Bhadrapada. It is kept at midnight, so the date is the day that tithi is running nishita, the middle of the night.
Computed from the Moon’s elongation with the Swiss Ephemeris, sidereal, Lahiri ayanamsa. A tithi begins and ends at a real minute rather than at midnight, which is why the moment above decides the day — and why the same tithi read at a different hour can name a different date.
Cast a free kundli, or ask any of 24 astrologers what this year actually holds for you. Billed by the minute, and only while somebody is answering.