So leap weeks are a thing. In the ISO week date system “week 1” is “the week with the year’s first Thursday in it”. Today, Monday Dec 29 2014, is actually ISO week 1 for the year 2015. How’s that for confusing?
Rumor has it this weird calendar math is responsible for Android Twitter being broken. Some part of Twitter’s servers were returning a date a year in the future, in the year 2015, which is causing invalid OAuth signatures because of an expired certificate or something like that. Presumably the engineer used the ISO week-year by accident.
This is a bug that keeps on giving; two years ago iPhones got stuck in Do Not Disturb mode, probably for the same bug.
In Unix date’s format language (strftime?) the weird year to avoid is %G. %Y does what’s expected.
$ date -u Mon Dec 29 15:47:54 UTC 2014 $ date -u +'%G-W%V-%u' 2015-W01-1