[Boost-users] boost::gregorian::date_period,how to increment/decrement the period

hello! boost::gregorian::days size(1); boost::gregorian::date_period tmp(period.begin()+=size,period.end()-=size); period=tmp; is there any better way to increment/decrement the period object I want period.begin+=size; period.end()-=size to be permanent to period but it doesn't,it returns an object... any idea?? thanks for any help.