I'm assuming from looking the code you're using Zend Framework. I've solved similar issues in Zend and Symfony using php partials.

You could create a partial _list_item.phtml

<li class="ui-content">
    <div style="float: left; width: 200px;">
        <span class="ui-icon ui-icon-trash ui-button ui-state-active" style="float: left; margin:3px;"></span>
        <input name="dates[]" class="datebox" style="width: 120px;" type="text" value="<?php echo $message ?>" />
    </div>
    <div style="padding-left: 200px;">
        <input name="contents[]" type="text" value="<?php echo $message ?>" style="width: 100%;" />
    </div>
</li>

Then in your view:

<?php foreach ($this->events as $event): ?>
    <?php echo $this->partial('list_item.phtml', array(
        'date' => $event->GetDate()->format('Y-m-d'),
        'message' => $event->GetMessage(),
    )); ?>
<?php endforeach; ?>

Then for your javascript:

function AddNewEvent()
{
    var today, html, temp;
    // call str_replace because javascript doesn't like new lines in strings
    html = '<?php echo str_replace(PHP_EOL, "\n", $this->partial('list_item.phtml', array(
        'date' => $event->GetDate()->format('Y-m-d'),
        'message' => '',
    ))); ?>';
    $(this).after(html);
    temp = $(this).next();
    CreateDateboxes($('.datebox', temp));
    $('.ui-icon-trash', temp).click(RemoveParent);
}

Viola! No template duplication because the affected code is in a partial. No excess javscript templating libraries, although if this kind of thing is prevalent in your application I'd suggest some refactoring and implementing such a templating library.

This approach will have the least impact on your work flow, little to no learning curve, and a short investment in development time for big decrease duplication (bosses love hearing that kind of thing).

I'm assuming from looking the code you're using Zend Framework. I've solved similar issues in Zend and Symfony using php partials.

You could create a partial _list_item.phtml

<li class="ui-content">
    <div style="float: left; width: 200px;">
        <span class="ui-icon ui-icon-trash ui-button ui-state-active" style="float: left; margin:3px;"></span>
        <input name="dates[]" class="datebox" style="width: 120px;" type="text" value="<?php echo $date ?>" />
    </div>
    <div style="padding-left: 200px;">
        <input name="contents[]" type="text" value="<?php echo $message ?>" style="width: 100%;" />
    </div>
</li>

Then in your view:

<?php foreach ($this->events as $event): ?>
    <?php echo $this->partial('list_item.phtml', array(
        'date' => $event->GetDate()->format('Y-m-d'),
        'message' => $event->GetMessage(),
    )); ?>
<?php endforeach; ?>

Then for your javascript:

function AddNewEvent()
{
    var today, html, temp;
    // call str_replace because javascript doesn't like new lines in strings
    html = '<?php echo str_replace(PHP_EOL, "\n", $this->partial('list_item.phtml', array(
        'date' => $event->GetDate()->format('Y-m-d'),
        'message' => '',
    ))); ?>';
    $(this).after(html);
    temp = $(this).next();
    CreateDateboxes($('.datebox', temp));
    $('.ui-icon-trash', temp).click(RemoveParent);
}

Viola! No template duplication because the affected code is in a partial. No excess javscript templating libraries, although if this kind of thing is prevalent in your application I'd suggest some refactoring and implementing such a templating library.

This approach will have the least impact on your work flow, little to no learning curve, and a short investment in development time for big decrease duplication (bosses love hearing that kind of thing).