• Motivation：Measuring and calculating time is crucial everywhere but many don't understand. Errors in time handling are harmful to science. There's a need for a centralized introduction as existing ones are wrong. This article aims to bootstrap correct understanding.
• Counting Seconds："Second" has a precise scientific definition as 9192631770 cycles of a specific quantum hyperfine transition in Caesium-133. A solar day is roughly 86400 seconds but Earth's rotational speed varies. Different time standards like TT, TAI, UT1, UTC, and Local Time have emerged due to this.
• Reference Frames：Imagine a perfect SI second clock. Due to General Relativity, gravity and velocity cause time dilation. The IAU defines three reference frames: TCB (at Solar System's barycenter), TCG (at Earth's center), and TT (on Earth's geoid). These clocks tick at different rates relative to each other. TT is most relevant for those on Earth.
• Coupling with Accurate Clocks：Atomic clocks are very accurate. Combining readings from multiple atomic clocks gives a better time scale called EAL. From EAL, TAI is derived by frequency steering. TAI is tied to the three reference frames at 1977-01-01 00:00:00.0000000. TT(TAI) drifts over time and BIPM publishes retrospective estimates.
• Making It Useful I: Calendars：There are three calendars to know - Julian, Traditional Gregorian, and ISO 8601 Gregorian. The Julian Calendar has a leap day every four years. The Traditional Gregorian Calendar corrects the Julian Calendar's leap year rules. The ISO 8601 Gregorian Calendar has days of exactly 86400 SI seconds and a year zero. Almost every country uses the ISO 8601 Gregorian Calendar.
• Making It Useful II: Tying to Astronomical Time Standards：To reconcile different ways of measuring a day's length, there are time standards like UT1 and UTC. UT1 measures Mean Solar Time at 0° longitude. UTC reconciles TAI with UT1 by adding/removing leap seconds. A leap second is a 61st second added at the end of a minute. ΔT reflects the Earth's rotational speed change.
• Making It Useful III: Offsets：Timezones are split based on UTC. Daylight Saving Time (DST) is added during summer. This leads to multiple time standards per UTC offset. Countries have added and changed timezones due to various reasons, causing a complex mess. IANA maintains the timezone database.
• Computerization: Leap Seconds and UNIX Time：UNIX Time (UXT) is the de-facto method for computer time measurement. It's a count of seconds since 1970-01-01 00:00:00 UTC but not SI seconds. UXT handles leap seconds by pretending they didn't happen, leading to poor software and glitches.
• Takeaways：When dealing with time in code, for different situations like getting the current time, recording an event, or timing an operation, different time standards should be used. Be as accurate as possible without necessarily being compatible.
• Summary：Discussed three reference frames tied to atomic clocks. Defined time and calendar systems related to Earth's rotation. Developed UTC to combine accurate timekeeping with useful time values. Local times are UTC conversions. On computers, UXT handles leap seconds poorly.
• Conclusion：Hopes the article is illuminating, informative, or useful. Welcomes corrections and feedback.