[Boost-users] Local_date_time constructor question

Documentation for local_date_time constructor local_date_time(...) Parameters: date time_duration time_zone_ptr bool states that the last parameter, "The DST flag", "must be passed to indicate whether the time is in daylight savings or not." My question is: Isn't the combination of the first parameter, "date" and third parameter, "time_zone_ptr" determine if the resulted local_date_time will be either in the daylight saving time period or not?