Michael Haubenwallner :

as i could not google out an answer for my problem with boost/date_time/dst_rules.hpp: is there a reason why "boost::date_time::dst_calculator::process_local_dst_start_day()" returns "invalid_time_label" for the transition time, but "process_local_dst_end_day()" returns "ambigous" ? Or is it just a bug ?

At the start of DST, the time goes directly from, say, 1:00 to 2:00, so there is *no* point in time labelled 1:30 and it is invalid. At the end of DST, the time goes directly from, say, 2:00 to 1:00, so there are two points in time labelled 1:30 and it is ambiguous.