Add an "Every 15 minutes" option to Limit Orders for WooCommerce

limit-orders-15min.php

<?php
/**
 * Plugin Name: Limit Orders for WooCommerce - 15min Intervals
 * Description: Add an "Every 15 minutes" option to Limit Orders for WooCommerce.
 * Author:      Nexcess
 * Author URI:  https://nexcess.net
 */

/**
 * Add "Every 15min" to the list of intervals.
 *
 * @param array $intervals Available time intervals.
 *
 * @return array The filtered array of intervals.
 */
add_filter( 'limit_orders_interval_select', function ( $intervals ) {
	// Return early if it already exists.
	if ( isset( $intervals['15min'] ) ) {
		return $intervals;
	}

	$intervals['15min'] = __( 'Every 15min (Resets every quarter hour)', 'limit-orders' );

	return $intervals;
} );

/**
 * Get a DateTime object representing the beginning of the current quarter-hour.
 *
 * @param \DateTime $start    The DateTime representing the start of the current interval.
 * @param string    $interval The type of interval being calculated.
 *
 * @return \DateTime A DateTime object representing the top of the current hour or $start, if the
 *                   current $interval is not "15min".
 */
add_filter( 'limit_orders_interval_start', function ( $start, $interval ) {
	if ( '15min' !== $interval ) {
		return $start;
	}

	return $start->setTime(
		(int) $start->format( 'G' ),
		floor( (int) $start->format( 'i' ) / 15 ) * 15, // Determine which quarter-hour we're currently in.
		0
	);
}, 10, 2 );

/**
 * Filter the DateTime at which the next interval should begin.
 *
 * @param \DateTime $start    A DateTime representing the start time for the next interval.
 * @param \DateTime $current  A DateTime representing the beginning of the current interval.
 * @param string    $interval The specified interval.
 *
 * @return \DateTime The DateTime at which the next interval should begin, or $start if the
 *                   current $interval is not "15min".
 */
add_filter( 'limit_orders_next_interval', function ( $start, $current, $interval ) {
	if ( '15min' !== $interval ) {
		return $start;
	}

	return $current->add( new \DateInterval( 'PT15M' ) );
}, 10, 3 );

Looking to use a custom interval other than 15min? This follow-up post in the WordPress.org forum breaks down the key lines in this plugin.