<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def set_start(self,time,pad = None):
 """ Set the start time of the datafind query. @param time: GPS start time of query. """

    if pad:
      self.add_var_opt('gps-start-time', int(time)-int(pad))
    else:
      self.add_var_opt('gps-start-time', int(time))
    self.__start = time
    self.__set_output()