Popup Maker Plugin: A basic example of scheduling a popup to display during a certain timeframe.

schedule-a-popup.php

<?php // Ignore this first line when copying to your child theme's functions.php file.

function schedule_my_popup( $is_loadable, $popup_id ) {
	if( $popup_id == 14 ) { // Change to your popup ID.
		date_default_timezone_set('Asia/Singapore'); // Change to your time zone.
		
		$now = strtotime( 'now' );
		  
		$start = strtotime( 'September 8, 2021 12:00PM' ); // Change to your local start time.
		  
		$end = strtotime( 'September 8, 2021 2:00PM' ); // Change to your local end time.

		$is_loadable = ( ( $now >= $start ) && ( $now <= $end ) ) ? 1 : 0;
	}
	
	return $is_loadable;
}
add_filter( 'pum_popup_is_loadable', 'schedule_my_popup', 2, 1000 );