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Under %W, the first week contains a full 7 days, but it only
contains 6 days under %U.

Either way, we end up with a week 53 - it contains 1 day for
%W, but 2 days for %U.

In both cases, December 31st is the Monday of week 53.

Unless I'm misunderstanding how this is meant to work, and
Week 1 is always the first full week of the year, with an
optional Week 0 before it (which would always have fewer
than 7 days, and doesn't exist at all if the year and the
week start on the same day).

If I *am* misunderstanding, then that's the bug in strptime
- it currently works in accordance with my understanding.