Please read the docs more carefully. You are using %W incorrectly.
You cannot use %W with %Y and reliably get useful results. %W
MUST be used in conjuction with %G.
From the docs:
The %U, %W, %L, %G, and %J formats are used to support the ISO-8601
format: YYYY-wWW-D. In this format, a date is written as a year, the
week of the year, and the day of the week. Technically, the week may be
considered to start on any day of the week, but Sunday and Monday are
the both common choices, so both are supported.
The %W and %G formats return the week-of-year and the year treating
weeks as starting on Monday.
The %U and %L formats return the week-of-year and the year treating
weeks as starting on Sunday.
Most of the time, the %L and %G formats returns the same value as the %Y
format, but there is a problem with days occurring in the first or last
week of the year.
The ISO-8601 representation of Jan 1, 1993 written in the YYYY-wWW-D
format is actually 1992-W53-5. In other words, Jan 1 is treated as being
in the last week of the preceding year. Depending on the year, days in
the first week of a year may belong to the previous year, and days in
the final week of a year may belong to the next year. The week is
assigned to the year which has most of the days. For example, if the
week starts on Sunday, then the last week of 2003 is 2003-12-28 to
2004-01-03. This week is assigned to 2003 since 4 of the days in it are
in 2003 and only 3 of them are in 2004. The first week of 2004 starts on
2004-01-04.
The %U and %W formats return a week-of-year number from 01 to 53. %L and
%G return the corresponding year, and to get this type of information,
you should always use the (%W,%G) combination or (%U,%L) combination. %Y
should not be used as it will yield incorrect results.
%J returns the full ISO-8601 format (%G-W%W-%w).
