TimeDelta &operator+=(const TimeDelta &t) { usec += t.usec; return *this; }
TimeDelta operator-(const TimeDelta &t) const { return TimeDelta(usec-t.usec); }
TimeDelta &operator-=(const TimeDelta &t) { usec -= t.usec; return *this; }
TimeDelta &operator+=(const TimeDelta &t) { usec += t.usec; return *this; }
TimeDelta operator-(const TimeDelta &t) const { return TimeDelta(usec-t.usec); }
TimeDelta &operator-=(const TimeDelta &t) { usec -= t.usec; return *this; }