Helper not working

[lcsqlpete]

I'm using the PHP version of Mustache and have defined a helper to convert a date in yyyy-mm-dd format to mm/dd/yyyy format which looks like this:

`'_mdy' => function ($ymd) {
            return date("m/d/Y", strtotime($ymd));
        }`

I am using it in a template like this
{{#concerts}} {{#_mdy}}{{ConcertStartDateTime}}{{/_mdy}} {{/concerts}}

All the dates from the concerts array are displayed as 12/31/1969 which I think is the value displayed when the date supplied to strtotime is not valid.

I added a line to the php script that assembles the data to execute the same command as in the helper ($concerts is the array supplied to the mustache renderer:
echo date("m/d/Y", strtotime($concerts[0]['ConcertStartDateTime']))
and it displayed the correct date.

Other values from the concerts array are displayed correctly.

What am I doing wrong?