Date and time explained

Articles on daylight saving time, leap years, ISO week numbers, Unix time and atomic clocks.

8 stories

Daylight saving time: why your meeting moved

Across the world 2 min read

A London–New York call is usually five hours apart. For a few weeks, it isn’t. Here is how to stop a calendar surprise.

Everyday puzzles 2 min read

Tomorrow is one date away. That does not always mean 86,400 seconds. The difference matters in code, travel, and everyday counting.

Calendar curiosities 2 min read

Week 53 is not a glitch. ISO week dates use a second kind of year, with its own boundary near New Year.

Calendar curiosities 2 min read

The exception to the exception explains February’s missing day—and why repeating the calendar every four years does not quite work.

Behind the clock 2 min read

A date, a local time, and an instant are three different things. Mixing them is how a harmless form loses a day.

Across the world 2 min read

An arrival time earlier than departure is not time travel. It is two local clocks telling one story.

Behind the clock 2 min read

Time does not stop. One particular way of storing it reaches its largest positive number.

Behind the clock 2 min read

The second is defined through an atom. Getting that definition onto your laptop is a different—and fascinating—problem.

Original explanations with worked examples. Reference links point to the underlying standards and documentation.