WooCommerce Buy One Get One 50% off through a set date

b1g150off.php

add_action('woocommerce_cart_calculate_fees', 'add_custom_discount_2nd_at_50', 10, 1 );
function add_custom_discount_2nd_at_50( $wc_cart ){
    if ( is_admin() && ! defined( 'DOING_AJAX' ) ) return;
	
	// Get out of here if we are past the specific date set in the dateDifference function below. 
	if ( 0 > tew_dateDifferenceEOM( '2021-06-30') ) return;
	
    $discount = 0;
    $items_prices = array();

    // Set HERE your targeted variable product ID
    $targeted_product_id = array( 524, 705, 833, 835 );

    foreach ( $wc_cart->get_cart() as $key => $cart_item ) {
        if( in_array( $cart_item['product_id'], $targeted_product_id ) ){
            $qty = intval( $cart_item['quantity'] );
            for( $i = 0; $i < $qty; $i++ )
                $items_prices[] = floatval( $cart_item['data']->get_price());
        }
    }
    $count_items_prices = count($items_prices);
    if( $count_items_prices > 1 ) foreach( $items_prices as $key => $price )
        if( $key % 2 == 1 ) $discount -= number_format($price / 2, 2 );

    if( $discount != 0 ){
        // Displaying a custom notice (optional)
        wc_clear_notices();
        wc_add_notice( __("You get 50% of discount on the 2nd item"), 'notice');

        // The discount
        $wc_cart->add_fee( 'Discount 2nd at 50%', $discount, true  );
        # Note: Last argument in add_fee() method is related to applying the tax or not to the discount (true or false)
    }
}
function tew_dateDifferenceEOM($date_eom, $differenceFormat = '%a' )
{
    $datetime1 = date_create($date_eom);
    $datetime2 = date_create( date("Y-m-d") );
   
    $interval = date_diff($datetime1, $datetime2);
   
    return $interval->format($differenceFormat);
   
}

Ref: https://stackoverflow.com/questions/46264431/woocommerce-discount-buy-one-get-one-50-off
Ref: https://www.php.net/manual/en/function.date-diff.php