code: func builtinTime(args []types.Datum, ctx context.Context) (d types.Datum, err error) {
  	if args[0].IsNull() {
  		return
  	}
  
  	str, err := args[0].ToString()
  	if err != nil {
  		return d, errors.Trace(err)
  	}
  	idx := strings.Index(str, ".")
  	fsp := 0
  	if idx != -1 {
  		fsp = len(str) - idx - 1
  	}
  	sc := ctx.GetSessionVars().StmtCtx
  	fspD := types.NewIntDatum(int64(fsp))
  	if fsp, err = checkFsp(sc, fspD); err != nil {
  		return d, errors.Trace(err)
  	}
  
  	return convertToDuration(sc, args[0], fsp)
  }
docs: // See http://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_time