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Scientific notation writes a number as a coefficient from 1 to under 10 times a power of 10, such as 4.5 × 10⁶. How to convert big and small numbers both ways, multiply, divide, add, read E notation in Excel and Python, and avoid common mistakes.

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.