Date and time reference guides

Date and Time Guides — Time Zones, Calendars and Timestamps

Understand why clocks change, how computers store time, and what makes a calendar date unambiguous. Practical explanations for everyday questions.

Time zone, calendar and timestamp guides

Read an explanation, work through an example, then try the related tool.

What Is UTC? A clear explanation of Coordinated Universal Time, why it exists, and why it is the reference layer behind time zones, timestamps, and software logs. What Is Unix Time? An original primer on Unix time, the 1970 epoch, why developers use it, and where it causes confusion in real systems. Daylight Saving Time Explained A practical guide to daylight saving time, why offsets shift, and why these transitions still break meetings, software, and scheduling workflows. ISO 8601 Explained A concise explanation of ISO 8601 date and time notation, why it is safer than ambiguous local formats, and where it appears in software. How Time Zones Were Invented An original historical explainer on why time zones were standardized and how transport, telegraph networks, and coordination problems forced the modern system into place. UTC vs GMT: What Is the Difference? UTC and GMT often show the same clock time, but they are defined differently. Learn the history, when each term is correct, and what to use in software. Military Time and the 24-Hour Clock Military time is the 24-hour clock. Learn to read and convert 12h and 24h formats, handle midnight and noon, and avoid the most common mistakes. What Is a Time Zone? Learn what a time zone really is, how offsets from UTC work, why they exist, why borders bend, and how your devices figure out which zone you are in. Time Zone Abbreviations Explained (EST, PST, CET and More) Why time zone abbreviations like EST, PST, CST, and IST are ambiguous, the collisions that cause real errors, and how to write times nobody can misread. What Are Leap Seconds? Learn what leap seconds are, why Earth's rotation is irregular, how UTC stays close to UT1, who decides, and why they will be retired by 2035. The IANA Time Zone Database (tz database) Discover the IANA time zone database: its history, the Region/City naming scheme, how it tracks historical rules, who uses it, and why offsets fail. How to Convert Between Time Zones Learn the reliable way to convert between time zones using UTC as a hub, with worked examples, DST pitfalls, and tips for scheduling global meetings. What Is the International Date Line? Discover the International Date Line: where it runs, why it zigzags, what happens when you cross it, and how it decides where each new day begins on Earth.

How to use date and time tools

A clock tells you the current time. Understanding its time zone tells you whether a colleague is awake. A timestamp records an event. Understanding its unit tells you whether you have found the right day or accidentally landed in 1970. These guides connect the numbers to the decisions you make with them.

Calendar dates, local times and instants

A calendar date names a day. A local time describes a clock reading in a place. An instant identifies one moment worldwide. Much of the confusion around travel, software, and scheduling comes from treating these as the same kind of information. Start with the guide closest to your task, then try its examples in the linked tools.