<form action="tes6.php" method="post">
<?php
  if (isset($_REQUEST['taste1'])) {
  unset($_REQUEST['Inhalt8']);}
?>
<?php
  if (isset($_REQUEST['taste2'])) {
  unset($_REQUEST['Inhalt10']);}
?>
<table>  
<tr>
<td>

  <input type="text" size="17" name="Inhalt8" value="<?php @print $_REQUEST['Inhalt8'];   if(isset($_POST['taste1'])){
                                                      $Inhalt8=date("d.m.y H:i:s");
                                                      echo "$Inhalt8";}
                                                      ?>"
  </td>
<td>
  <input type="submit" name="taste1" value="OK1">
  </td>
</tr>
<tr>
  <td>
  <input type="text" size="17" name="Inhalt10" value="<?php @print $_REQUEST['Inhalt10'];  if(isset($_POST['taste2'])){
                                                      $Inhalt10=date("d.m.y H:i");
                                                      echo "$Inhalt10";}
                                                      ?>" 
 </td>
    <td align= "center">
  <input type="submit" name="taste2" value="OK2"/>
 </td>

  </tr>
</table>
  </form>