getpaid-dynamic-trial-periods.php

<?php

// Give free trials until 2023-09-02.
$GLOBALS['wpinv_trial_ends'] = '2023-09-02';

if ( time() < strtotime( $GLOBALS['wpinv_trial_ends'] ) ) {

    // Allow free trials.
    add_filter(
        'wpinv_get_item_is_free_trial',
        function() {
            return 1;
        }
    );

    // Set trial period to days.
    add_filter(
        'wpinv_get_item_trial_period',
        function() {
            return 'D';
        }
    );

    // And the interval as the number of days to 2023-09-02.
    add_filter(
        'wpinv_get_item_trial_interval',
        function( $interval ) {

            if ( time() > strtotime( $GLOBALS['wpinv_trial_ends'] ) ) {
                return $interval;
            }

            // Calculate number of days from now to 2023-09-02
            $now         = new DateTime();
            $future_date = new DateTime( $GLOBALS['wpinv_trial_ends'] );
            $interval    = $future_date->diff( $now );

            return $interval->days + 1;
        }
    );
}