Conditional output in templates based on date?

I'm trying to customise search results at the template stage. What I'd like to achieve is for the result to only appear if a date field in the result is less than or equal to the current date.

I'm passing in the current date from search.php like this:

PerchSystem::set_var('current-date', date('Y-m-d'));

And I'm using Perch:if statements to try and control the display of the results like this (where item_post_date is a date formatted in Y-m-d):

<perch:if id="item_post_date"  match="lte" value="current-date">
    ... item output code here
<perch:else>
    <p>Item not yet published
</perch:if>

The problem is, the item is always displayed. Is it actually possible to compare these two variables in the template this way, or do I need to use an alternative date format?