I thought I found a bug in the cal(1) command, in how it numbered weeks. Today "cal -w" shows week 5, where my other (work) calendar shows week 4.
Turns out it's not a bug, but it's much worse.
In the US "Jan 1 is always First week", so a week is actually assigned two numbers, depending on the year. In ISO-8601, a week only has one number (so Jan 1 2021 was week 53).
See "cal -wm"
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