There's Always Something Worth Discovering.

Roman numerals use seven letters: I, V, X, L, C, D and M. The adding and subtracting rules, a 1 to 100 chart, step-by-step conversion, years like MMXXVI, where they are used today and the most common mistakes.

A year is a leap year if it divides by 4, except century years, which must also divide by 400. The three-step check, why 2100 is skipped, the 1582 and 1752 calendar changes, a 2000-2100 list, and the Excel 1900 bug.

The 24-hour clock numbers hours from 00 to 23, so 14:30 is 2:30 p.m. A full conversion chart, two quick tricks, what NIST and ISO 8601 say about noon, midnight and 24:00, and how to switch formats on phones, in Excel and in code.

A Unix timestamp is the number of seconds since 00:00:00 UTC on 1 January 1970. How to read it, tell seconds from milliseconds, convert it in Excel, Sheets, Python, JavaScript, MySQL and the shell, and what the 2038 limit means.

Daylight saving time moves clocks forward one hour in spring and back in autumn. US, UK and EU rules and dates, places that skip it, how it changes time gaps with India, its history, energy and health evidence, and whether it is ending.