[boost] [interprocess] Timed locks

In boost::inteprocess timed locks are specified by giving a boost::posix_time::ptime as the absolute time until which ownership is attempted. Would it not be more usable to give a boost::posix_time::time_duration from the time in which the one attempts to acquire the lock ? I think an end-user calculation is usually one in which he says that a certain lock needs to be acquired in so many seconds ( or milliseconds ) and a boost::posix_time::time_duration would fit that calculation better.