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Short, plain-English explanations of how our calendars and clocks work — with the numbers worked out from the same checked data as our tools.
Why does Easter move every year?Easter is the first Sunday after the spring full moon, so it can fall from 22 March to…Why do we have leap years?A year is not exactly 365 days, so every four years we add 29 February — except in most…How do week numbers work?Week 1 is the week with the year's first Thursday. Here is the ISO 8601 rule, why some…Why do the clocks change?Why we put the clocks forward in spring and back in autumn, when it happens in Europe,…How do time zones work?What UTC and GMT are, why time zones are offsets like UTC+5:30, which places use half-…How does the Islamic calendar work?The Hijri calendar follows the moon: 12 months of 29 or 30 days, so the year is about 11…How does the Chinese calendar work?Months start at the new moon and a leap month keeps the Chinese calendar in step with…How does the Hebrew calendar work?The Jewish calendar has lunar months and a leap month seven times in 19 years, so…Why isn't the earliest sunset on the shortest day?In London the earliest sunset is around 12 December and the latest sunrise around 30…Why does the moon have phases?Why the moon's phases repeat every 29½ days, why some years have 13 full moons, and when…