For more details on conversion specifiers please refer to
the manual page of strptime. Most of the formatters
of the form %x, where x is a letter, are supported.
Moreover, each %% is replaced with %.
Warnings are given in case of %x, %X, %u, %w,
%g, %G, %c, %U and %W
as in such circumstances either ICU does not
support the functionality requested using the format-strings API
or there are some inconsistencies between base R and ICU.