Guides

The reasoning behind the tools: what the numbers mean, where the rules come from, and the cases where the obvious answer is wrong.

What the guides cover

Each tool on this site gives an answer. These guides explain the answer: why a Unix timestamp is the same in every zone, why 31 January plus a month is not obvious, why a week number can belong to a different year from the date, and why some countries keep their clocks 45 minutes off the hour. They are written from the same sources the tools are built on, the IANA time zone database, the ISO 8601 standard and the NOAA solar equations, and each one links to the calculator that does the work.

Every guide is dated and signed. When a rule changes, such as a country abolishing daylight saving, the guide that depends on it is revised and the revision date updated. Corrections are welcome through the contact page.

All guides

What Unix time is, and what it is not What a Unix timestamp counts, why it is the same in every time zone, how leap seconds and milliseconds fit in, and where 32-bit systems run out.
The year 2038 problem What happens at 03:14:07 UTC on 19 January 2038, which systems are still exposed, how the fix works, and why the problem shows up long before the date itself.
ISO 8601, the date format that sorts itself How to read and write ISO 8601 dates, times, offsets, week dates, ordinal dates and durations, what the Z means, and how RFC 3339 differs.
How ISO week numbers work The rule that decides week 1, why some years have 53 weeks, why 29 December can fall in next year’s week 1, and how ISO weeks differ from the US system and Excel.
How daylight saving time works What happens to the clock when DST starts and ends, why one night has 23 hours and another 25, which countries change and when, and the mistakes that follow.
The time zones that are not on the hour The zones at UTC+05:30, +05:45, +08:45, +12:45 and the rest: where they are, how each came about, and why software should never assume whole-hour offsets.
UTC and GMT: the same time, for different reasons Why UTC and GMT are usually treated as the same, where they genuinely differ, what leap seconds have to do with it, and which one to write in software.
How to schedule a meeting across time zones How to find hours that work for two or three cities, why some pairs have no overlap, the weeks when DST breaks standing meetings, and how to write a time clearly.
The IANA time zone database, explained What tzdata is, why zones are named after cities, why abbreviations like CST are ambiguous, how often the rules change, and how the data reaches your browser.
How to store dates and times correctly When to store UTC, when to store local time with a zone name, why offsets alone are not enough, which database types to use, and the parsing traps in JavaScript.
How age is calculated, including the awkward cases The rule most countries use, what happens to people born on 29 February, why age in months and days needs care, and how East Asian age counting differed.
How business days are counted How working days are counted, inclusive and exclusive conventions, what a weekend holiday does, where the weekend is not Saturday and Sunday, and Excel’s functions.
Leap years, and why 2100 will not be one Why February gets a 29th day, the divisible-by-4 rule and its two exceptions, why 1900 was not a leap year but 2000 was, and how the Gregorian reform happened.
The International Date Line Why the date line zigzags, how Kiribati and Samoa moved it, which places see the new year first and last, and why the world spans 26 hours rather than 24.
How sunrise and sunset are calculated The NOAA equations behind every sunrise table, why the sun is up before it has risen, the three twilights, and why the earliest sunset is not on the solstice.
Adding a month to a date, and why the answer varies What happens when the target month is too short, how software clamps or overflows, why month arithmetic does not add up, and how billing dates should be defined.