Your festival, eight thousand kilometres later

The Indian calendar counts lunar days. A lunar day is called a tithi, and it is one step in the moon's separation from the sun. Thirty steps make a lunar month. Diwali is kept on the last step of the dark fortnight, the one called amavasya, which ends at the instant the moon passes the sun.

That instant is a fact about the solar system. It happens once, for everybody, and every almanac on earth works from the same number for it.

In 2027, amavasya begins on Thursday 28 October at 8.47 in the evening by Indian clocks and ends the next evening at 7.05. Indian clocks run nine and a half hours ahead of New York clocks that week. So the very same two moments are 11.17 on Thursday morning and 9.35 on Friday morning in New Jersey.

Now the local part. Diwali lamps are lit in the stretch just after sunset, so the almanac asks one question. On which evening was amavasya running after the sun went down?

In Delhi the sun sets at 5.39 on both evenings. On Thursday, amavasya has not started; it begins three hours later. On Friday it is running until 7.05. Delhi keeps Friday.

In New Jersey the sun sets just before six. On Thursday, amavasya has been running since late morning and will continue all night. On Friday it finished before lunch. New Jersey keeps Thursday.

Nobody has made a mistake. The two sheets asked the same question about two different sunsets, and the boundary of the lunar day happened to fall in the 10 hours between them.

The practical part comes at the end. Every almanac app has a city setting, usually fixed once and then forgotten. If it says Delhi and you live in New Jersey, it is handing you Delhi's evenings. A family or a temple that picks a date on purpose is keeping time properly. Keeping a date nobody chose is the thing worth avoiding.