I rediscovered this on 3/14/20 after having disappointing experience with TimerSys Popup / WP Popups.
I like!

Extensions page

Date Conditions Hack, at Creatures

Here’s my code for custom date window condition:
NOTE: Wish to heck I had documented where I found the clues for doing this…
It may be this one…

function pum_time_window_condition( $conditions = array() ) {
    return array_merge( $conditions, array(
        'date_win' => array( // The ID for the condition
            'group'    => __( 'General', 'popup-maker' ), // The section within the conditions drop-down to add this condition to.
            'name'     => __( 'Date Range', 'popup-maker' ), // The name that appears to admins when selecting the condition.
            'priority' => 10, // The priority determines the order the conditions are listed in within the group. Lower appears higher in the list.
            'callback' => 'pum_time_window_condition_callback', // The function to call to determine the state of the condition.
            'fields'   => array(
				array('label' => 'Start Date (YYYY-MM-DD)', 'name' => 'start_date', 'type' => 'text'),
				array('label' => 'End Date (YYYY-MM-DD)', 'name' => 'end_date', 'type' => 'text'),
			), 
			// If you need to show a text box/drop-down to get input from the admin, you can pass fields here.
        ),
    ));
}
add_filter( 'pum_registered_conditions', 'pum_time_window_condition' );

And the callback:

/**
 * Determines if today is within the date window
 * @param array $condition The condition settings, including input into any `fields`, if registered with the condition.
 * @param array $popup The current popup that is being checked.
 * @return bool
 */
function pum_time_window_condition_callback( $condition, $popup ) {
	if ( !isset($condition) ) {
		return true;
	}
	$today = date('Y-m-d');
    if ( $today >= $condition[0] && $today <= $condition[1] ) {
        return true;
    }
    return false;
}