Kṛṣṇa Janmāṣṭamī — and the smārta / vaiṣṇava split, stated
The popular claimTwo dates for one birth, every few years, and both sides are following a rule. In 2027 they differ.
This is the one festival where the question "which day is it" honestly has two answers, and neither side is careless. Here is what each is doing.
What the text gives
The Bhāgavata Purāṇa, tenth book, third chapter: the birth in the month of Bhādrapada, the dark half, the eighth tithi, the nakṣatra Rohiṇī, at the dead of night. Aṣṭamī and Rohiṇī together at midnight is the ideal the rules are built around. It does not occur every year, and when it does not, the two traditions choose different things to keep.
The smārta rule
Take the day whose niśītha — the midnight muhūrta — falls in Aṣṭamī. If both nights qualify, the second. If neither, the one with Rohiṇī at niśītha. If neither has Rohiṇī, the second. The tithi is primary; the nakṣatra decides ties. This is the reading of the Dharmasindhu and of most household calendars in the north.
The vaiṣṇava rule
Take the day on which Aṣṭamī is present at sunrise, and among candidate days prefer the one with Rohiṇī, even at the cost of the tithi being a little past by midnight. The vaiṣṇava saṁpradāyas — and the temples at Mathurā and Vṛndāvana — follow this, and it is why their date sometimes lags the smārta one by a day. The principle is that a tithi is observed on the day it touches sunrise, as for most vratas; the smārta side answers that a birth at midnight should be kept at midnight.
2027, computed
Aṣṭamī begins at 8:25 PM on Tuesday 24 August and ends at 7:20 PM on Wednesday 25 August. Rohiṇī begins at 12:28 PM on Wednesday 25 and ends at 11:42 AM on Thursday 26 — Delhi, this site's engine.
Smārta. Tuesday night's niśītha, around midnight going into the 25th, is in Aṣṭamī: the tithi began at 8:25 that evening. Wednesday night's niśītha is not: Aṣṭamī ended at 7:20 PM. One night qualifies. Rohiṇī is absent from it — the Moon is in Kṛttikā until Wednesday noon — but the tithi rule is primary. Smārta Janmāṣṭamī 2027: the night of Tuesday 24 August.
Vaiṣṇava. Aṣṭamī is present at sunrise on Wednesday the 25th (5:56 AM; the tithi runs until 7:20 PM). It was not present at sunrise on Tuesday. Rohiṇī arrives on Wednesday at 12:28 PM and holds through Wednesday night. Aṣṭamī at sunrise and Rohiṇī on the same day: the vaiṣṇava rule's two conditions both point at Wednesday. Vaiṣṇava Janmāṣṭamī 2027: Wednesday 25 August.
So in 2027 the two traditions keep consecutive days. The smārta side keeps the midnight that was in Aṣṭamī; the vaiṣṇava side keeps the day that had Aṣṭamī at dawn and Rohiṇī by noon. Each is applying its stated rule exactly.
Why the split is not a mistake
It would be tidier if one side were simply wrong. Neither is. The Bhāgavata gives a birth with three coordinates — tithi, nakṣatra, hour — and the sky does not always offer all three at once. A rule has to say which to give up first. The smārta rule gives up the nakṣatra; the vaiṣṇava rule gives up the hour. That is the entire disagreement, and it is a disagreement about priority, not about fact. A calendar that prints one date without saying which rule produced it has hidden the interesting part.
Why the times above are not the times for you
Niśītha is computed from your own sunset and sunrise; the sunrise test uses your own sunrise. A city nine and a half hours behind Delhi will find Aṣṭamī's 8:25 PM start at 10:55 AM local on the 24th — present at neither Tuesday's dawn nor, possibly, its midnight, depending on the night's length. This site's panchang runs both rules for the city you type and shows both answers when they differ.
What this page does not say
It does not say which tradition is right. It does not say that keeping the other day is keeping the wrong one. It states two rules, cites their sources, computes both, and prints both — which is the only honest thing a calendar can do with this festival.
Source — Bhagavata Purana 10.3 on the birth at midnight with the Moon in Rohini; Dharmasindhu and Nirnayasindhu on the smarta rule (nishitha-vyapini ashtami) and on the vaishnava rule (udaya-vyapini ashtami with Rohini preferred); tithi, nakshatra and sunrise computed by this site's panchang engine for Delhi
Every claim on this site names its source, and every calculator shows its arithmetic.