Add code to the top of the page just below the recordset, or at least somewhere above the loop like:
<?php $writeDate = strtotime($row_Recordsetcustomers['date']); ?>
Yeah I based the code on the assumption that your code was basically working. I see now that you had a flaw in your logic when incrementing the date in general:
<input name="hf_date_<?php echo $RepeatSelectionCounter_1; ?>" type="hidden" id="hf_date_<?php echo $RepeatSelectionCounter_1; ?>" value="<?php echo date ( 'Y-m-d H:i:s' , $writeDate); ?>" />
<?php
$writeDate= strtotime ( '+ '.$row_Recordsetcustomers['frequency'].' day' , date('Y-m-d H:i:s' , $writeDate) );
?>
(incidentally I don't have time to test this, so you may need to do a little debugging if you run into problems. The concepts should be sound)