Why two panchangs disagree

Two panchangs (printed Indian almanac sheets) can give different dates for the same festival. Four honest reasons, all of them bookkeeping.

First, the city. A panchang is not computed for India. It is computed for a place. The day begins at sunrise, and sunrise is local. The sun reaches Guwahati about an hour and a quarter before Ahmedabad, and both keep Indian Standard Time. If a lunar day changes over inside that gap, the two cities are genuinely on different dates. A smaller version sits inside a single city. Publishers differ by minutes over what counts as sunrise. Some measure from the sun's top edge, some from the middle of the disc, and some allow for the air bending the light into view early.

Second, the rule. A tithi, a lunar day, does not fit tidily inside a calendar date, so it must be assigned to one. What most people never learn is that there is more than one rule, and which one applies depends on the festival. Some go by sunrise, some by midday, some by the stretch after sunset, some by midnight. Ganesh Chaturthi is settled by the middle of the day, so it can land on a date the sunrise test rejects. Somebody applying the sunrise rule from memory is not careless, just using the wrong rule for that festival.

Third, the month name. Some regions end the lunar month at the new moon (the amanta reckoning), others at the full moon (the purnimanta reckoning). The two agree for half of every month and differ by one month name for the rest. Two households keep the same night and write a different word at the top of the page. When relatives argue about the month, ask first whether they disagree about the day. Often not.

Fourth, the numbers. Older almanacs work from tables handed down in memorised verse. Newer ones compute positions to match what is observed, which is what drk ganita, reckoning by sight, means. Same rules, different figures, and over centuries the old tables drifted from the sky enough to matter at boundaries.