● Guides & explainers

The ZenCountdown Blog

Plain-language explainers on how date and time math actually works, plus practical guides for planning with dates — the ideas behind our calculators, written out.

How ISO Week Numbers Actually Work (And Why January 1 Isn't Always Week 1)

ISO 8601 week numbers explained: why week 1 is defined by January 4, why some years have 53 weeks instead of 52, and worked examples using real dates from 2026 and 2027.

August 23, 2026