code: public function add(Time $t)
      {
          // Interval to add as days
          $td = $t->days;
  
          // Days (jda) and day fraction (dfa) to add
          $jda = intval($td);
          $dfa = $this->dayFrac + $td - $jda;
  
          // Handle the event that the day fraction becomes negative
          if ($dfa < 0) {
              $dfa += 1;
              $jda -= 1;
          }
  
          // Additional day to add from above day frac in excess of 1 day
          $jda1 = intval($dfa);
  
          // Since additional day has been added, modulate day frac to range 0-1
          $dfa = fmod($dfa, 1);
  
          // Add the intervals
          $this->jd      = $this->jd + $jda + $jda1;
          $this->dayFrac = $dfa;
  
          return $this;
      }
docs: Adds a Time interval to this instance
  
  @param  Time $t
  
  @return static