Practical guides that explain the concepts behind our tools, with real examples.
How many square meters is a sào or a tsubo, how many grams is a lượng of gold or a Chinese jin? Exact values, regional differences and the surprising links between Vietnamese, Chinese, Japanese and Korean units.
A practical guide to QR codes: their structure, versions and capacity, error correction levels L/M/Q/H, Wi-Fi and vCard formats, and design rules for codes that scan reliably on screen and in print.
Learn the five fields of a cron expression, special characters like *, /, - and comma, common schedules, and the pitfalls that make cron jobs run at the wrong time.
Why fast hashes like MD5 and SHA-256 are the wrong tool for passwords, how salts and work factors protect users, and which algorithm (Argon2id, bcrypt, scrypt, PBKDF2) to choose in 2026.
UUID v4 is random, UUID v7 is time-ordered. Learn how each is built, why v4 can hurt database index performance, and when v7 (or ULID) is the better primary key.
What a Unix timestamp is, seconds vs milliseconds, converting timestamps in JavaScript, Python and SQL, time zone best practices, and how the Year 2038 problem can still break your software.
