'Subtract two Epoch\'s or a Duration from an Epoch.
Valid:
Duration = Epoch - Epoch
Epoch = Epoch - Duration
= INPUT VARIABLES
- rhs    The Epoch to subtract.
= RETURN VALUE
- Returns either the duration between to Epoch\'s or the a new
Epoch that is the result of subtracting a duration from an epoch.'

def __sub__(self, rhs):

    import matplotlib.testing.jpl_units as U
    if isinstance(rhs, U.Duration):
        return (self + (- rhs))
    t = self
    if (self._frame != rhs._frame):
        t = self.convert(rhs._frame)
    days = (t._jd - rhs._jd)
    sec = (t._seconds - rhs._seconds)
    return U.Duration(rhs._frame, ((days * 86400) + sec))
