The ISO week-year notation can produce confusing values
when the first week of the year is so short that it
becomes week 0 and is considered the last week of the
previous year, depending on the locale.
For instance, using ISO notation:
'W53 2015' == dates.format_date(
date(2015,1,1), format="'W'w YYYY", locale='en_GB')
'W53 2005' == dates.format_date(
date(2006,1,1), format="'W'w YYYY", locale='de_DE')
This is surprising but actually valid.
However it definitely yields wrong output when combined with
months formats:
'January 2014' == dates.format_date(
date(2015,1,1), format="MMMM YYYY", locale='en_GB')
As a result we must always use `y` to denote the year in
any date format, *except* when it is combined with the
week number `w`, in which case we must use `Y`.
See the documentation at:
http://babel.pocoo.org/docs/dates/#date-fields
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