[boost] [date_time] timezone database

I believe others have pointed out that the time zone database would be much more useful if for each time zone one could have multiple daylight saving rules, each with its own start and stop time of applicablity. It seems to me that the easiest way to do this would be to use the files generated from the zoneinfo files by zic. A time zone database would be specified by the path to a directory structure containing one or more files produced by zic. On many Unix-style systems these files already exist, and the tool (zic) exists to produce them for others.