boost::local_time::operator-=
// In header: <boost/date_time/local_time/date_duration_operators.hpp> local_date_time operator-=(local_date_time & t, const boost::gregorian::months & m);
Subtracts a months object from a local_date_time. Result will be same day-of-month as local_date_time unless original day was the last day of month. see date_time::months_duration for more details
