304 Not Modified not working

I haven’t been able to get the browser caching to work.
I’m running the website on Windows OS with IIS7.5 and PHP 5.4.32.
I’ve tracked down the problem to the last condition in wp-cache-phase1.php:

if ( $remote_mod_time != 0 && $remote_mod_time == $local_mod_time ) {
header(“HTTP/1.0 304 Not Modified”);
exit();
}

The first condition fails because $remote_mod_time = ‘Sat, 07 Feb 2015 15:27:37 GMT’.
If I change the code to this then everything works as expected:

if ( $remote_mod_time && $remote_mod_time == $local_mod_time ) {
header(“HTTP/1.0 304 Not Modified”);
exit();
}

https://wordpress.org/plugins/wp-super-cache/