[Koha] help wanted in "Edit the joining date"

Hi, I am working on koha 2.2.9. I have a little Problem about the joining date. if i want to edit the joining date and save it, when edit a patron, can u help me. I mean that for the first time i have add a patron, the joing date i entered was wrong and i want to edit it again and add the correct date, how should i do it? I have also made a little change in the file "memberentry.tmpl" and modify the line <th>Joining date: </th> <td colspan="4"><!-- TMPL_VAR name="joining" --></td> modified line <input type="text" name="joining" size="12" maxlength="10" value="<!-- TMPL_VAR name="joining" -->" /> this shows me the text box of joining date but changes are not saved in database. can some one guide me. Regards Zahid