Adding week number support for time axis

Format specifiers '%U' and '%W' now behave like the specifications in the Open Group's implementation of strftime - they represent week numbers of dates [00-53] for weeks starting on Sunday and Monday respectively. In any calendar year, the first week that contains a Sunday (or Monday) is numbered Week 1. Any days before that fall in Week number 0 (which can have at most six days). This functionality respects custom timezones set in the options.