Code working just in specific time period

You could filter body_class and add a time depending class:

add_filter( 'body_class', 'time_body_classes' );

function time_body_classes( $classes )
{
    $classes[] = 'time-' . date( 'a' ); // time-am or time-pm
    $classes[] = 'time-' . date( 'H' ); // time-02 or time-17

    return $classes;
}

Then you can make the CSS selector more specific:

.time-am iframe {
}
.time-02 iframe,
.time-03 iframe,
.time-04 iframe,
.time-05 iframe 
{
}

Be aware the body classes will not change if you use a caching plugin.