Why does subtracting time return 1 instead of 0 after echo cancellation

I have this table in my opinion

  duty   |  exact   |  undertime
12:00:00 | 12:00:00 | 01:00:00

      

        
        
        
      

    

period is the result $hour = $duty - $exact

, while i

echo date("H:i:s",$hour);

      

        
        
        
      

    

it repeated 01:00:00 expecting 00:00:00. What did I miss?

here is my code, I am using foreach because the debt and exact data is from my database query.

<?php foreach($unique_date as $unique_dates):?>  
                <tr>
                  <td align="center" colspan="2">
                      <?php echo date($unique_dates['dtr_date']);?>
                  </td>
                  <td align="center" colspan="2">
                      <?php $timediffs = $unique_dates['timediffs'];
                            echo $timediffs;
                      ?>
                  </td>
                  <td align="center" colspan="2">
                      <?php $exact_total_sched = $unique_dates['exact_total_sched'];
                            echo $exact_total_sched;
                      ?>
                  </td>
                  <?php                         
                        $duty = strtotime($timediffs);
                        $exact = strtotime($exact_total_sched);
                  ?>
                  <?php
                        if($duty < $exact)
                        {
                          echo "<td align=\"center\" colspan=\"2\">";
                                #UNDERTIME  
                                $hour1 = $exact-$duty;
                                echo date("H:i:s",$hour1);
                          echo "</td>";
                          echo "<td align=\"center\" colspan=\"2\">";
                          echo date("00:00:00");
                          echo "</td>";
                        }
                     else if($duty > $exact)
                       {
                          echo "<td align=\"center\" colspan=\"2\">";
                           echo date("00:00:00");
                          echo "</td>";
                          echo "<td align=\"center\" colspan=\"2\">";
                                #OVERTIME
                                $hour2 = $duty-$exact;
                               echo date("H:i:s",$hour2);
                          echo "</td>";
                       }
                      else if($duty == $exact)
                       {
                          echo "<td align=\"center\" colspan=\"2\">";
                                $hour3 = $duty-$exact;
                          echo date("H:i:s",$hour3);
                          echo "</td>";
                          echo "<td align=\"center\" colspan=\"2\">";
                          echo date("00:00:00");
                          echo "</td>";
                       }
                      ?> 
                </tr>

              <?php endforeach ?>