Which places use half-hour time zones, and what breaks?

Every city in our dataset on a non-whole-hour offset, with January overlap figures. None of these three zones observes daylight saving, so the offsets hold all year.

Why these offsets exist

India adopted a single national time in the 1900s and picked the meridian at 82°30′ east, roughly through Allahabad, because a country that wide had to choose between one awkward zone and two convenient ones and chose unity. Sri Lanka has moved back and forth several times and has settled on the Indian offset.

Nepal’s fifteen-minute difference from India is deliberate. It is set to the meridian through Gaurishankar, a mountain east of Kathmandu, and it dates from a period when distinguishing Nepali time from Indian time carried real political weight. It is not an accident and it is not going away.

Elsewhere in the world the same pattern appears in Iran, Afghanistan, Myanmar, parts of Australia, Newfoundland and the Chatham Islands. None of those appear in our city dataset, but if you deal with clients there the same problems apply.

What actually breaks

Mental arithmetic. This is the real cost and it is larger than it sounds. Whole-hour differences can be done in your head while doing something else. Half-hour differences cannot, at least not reliably, and the failure mode is a meeting missed by thirty minutes rather than an hour, which is the harder one to explain.

Rounding in software. A worrying number of tools store offsets as integers. The symptom is a display that says UTC+6 for a city that is actually UTC+5:30, or an overlap calculator that reports whole hours when the true answer is two and a half. We hit this ourselves while building these pages, which is why every figure here is computed from the time zone database rather than from a stored offset.

Calendar invites written in the wrong direction. If you write a recurring meeting in your own local time and your client is on a whole-hour zone, their copy sits on a :30 forever and slowly gets moved by other people booking around it. Write recurring meetings in the client’s time zone.

The overlap is genuinely good, which is the point

It is worth saying that the awkwardness buys you something. Mumbai at $1,300 a month gives 2h of a London working day in January and 4h in July. Colombo and Galle are the same offset at $1,000 a month.

That is a better European overlap than anywhere in Southeast Asia, at prices comparable to Vietnam. Ella and Kandy in the Sri Lankan hill country come in at $900. Nepal is cheaper again — Pokhara at $700 and Dharan at $600 are among the lowest monthly costs anywhere in our dataset.

A rule that makes it survivable

Pick one anchor city and think in its clock. If most of your work is British, put London on your phone’s second clock and stop converting. Your own local time becomes the thing you look up, which sounds backwards and is much less error-prone, because you are only ever converting when you are physically in the room with a clock to check.

The people who make half-hour zones work all do some version of this. The people who struggle are the ones still doing the sum in their head every time, six months in.