World Clock

Five cities, one glance

A world clock answers one question continuously: what time is it, right now, for the people I work with? The default view here shows India, London, New York, Dubai and Singapore — a spread that covers the working day of most Indian teams with international clients.

The value is not the arithmetic, which is simple. It is avoiding the specific errors that come from doing it in your head: forgetting that one city has moved its clocks, forgetting that today is the day another one moves, or misremembering an offset by an hour in the direction that makes you late.

How the times are derived

Every zone is resolved from the IANA time zone database, the reference dataset maintained collaboratively and shipped in essentially every operating system and programming language. Each city maps to a zone identifier such as `Asia/Kolkata` or `America/New_York`, and the database holds the full history and future schedule of offsets and daylight saving rules for each.

The current time in any zone is then:

Local time = current UTC instant + that zone's offset for this instant

The phrase *for this instant* is the important part. An offset is not a property of a place; it is a property of a place at a moment. New York is UTC−5 in January and UTC−4 in July. A calculation using a fixed offset is right for part of the year and wrong for the rest.

India's unusual offset

India runs at UTC+5:30, one of a small number of zones on a half-hour offset. Others include Iran (UTC+3:30), Afghanistan (UTC+4:30), Myanmar (UTC+6:30) and parts of Australia (UTC+9:30). Nepal is on UTC+5:45, a quarter-hour offset shared with almost nowhere else.

India also uses a single time zone across the entire country, spanning roughly 30 degrees of longitude. The practical consequence is a substantial difference in solar time between the far north-east and the west — sunrise in Arunachal Pradesh can be more than an hour and a half ahead of Gujarat while both read the same clock. Proposals for a second zone have been debated for decades without adoption.

India does not observe daylight saving, which means its offset is constant while many of its trading partners' offsets shift twice a year.

The DST asymmetry that catches Indian teams

Because India is fixed and Europe and North America are not, the gap to those cities changes twice a year even though nothing changed at the Indian end.

London is 4.5 hours behind India in winter and 3.5 hours behind in summer. New York is 10.5 hours behind in winter and 9.5 hours in summer. A recurring 8 p.m. call with New York moves to 7 p.m. or 9 p.m. IST twice a year, depending on which end scheduled it.

Worse, the transition dates do not align. The EU changes on the last Sunday in March and October; the US on the second Sunday in March and the first in November. For several weeks each year the gap between London and New York is not its usual five hours, and meetings scheduled across all three zones drift unpredictably.

Common mistakes

Memorising an offset. It is valid for half the year at best.

Assuming everyone changes clocks on the same date. The US and EU differ by several weeks.

Forgetting the southern hemisphere runs opposite. Sydney's DST begins as Europe's ends.

Using a country name instead of a zone. The US spans six zones; Australia several. "US time" is not a time.

Trusting a device with a manually set clock. A world clock is only as accurate as the system time underneath it.