docs: def invoke (self, args, **kwargs):
          """c2m  - UTC calendar to MJD[TAI]"""
  
          if len (args) not in (3, 6):
              raise multitool.UsageError ('c2m expected exactly 3 or 6 arguments')
  
          year = int (args[0])
          month = int (args[1])
  
          import astropy.time
  
          if len (args) == 3:
              day = float (args[2])
              iday = int (math.floor (day))
              r = 24 * (day - iday)
              hour = int (np.floor (r))
              r = 60 * (r - hour)
              minute = int (np.floor (r))
              second = 60 * (r - minute)
          else:
              iday = int (args[2])
              hour = int (args[3])
              minute = int (args[4])
              second = float (args[5])
  
          s = '%d-%02d-%02d %02d:%02d:%02.8f' % (year, month, iday,
                                                 hour, minute, second)
          t = astropy.time.Time (s, format='iso', scale='utc')
          print ('%.4f' % t.tai.mjd)