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 the 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 string format API
or there are some inconsistencies between base R and ICU.