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