<?php
    $tz_input = $_GET['tz'] ?? 'UTC';
    $timezone = in_array($tz_input, timezone_identifiers_list())
        ? new DateTimeZone($tz_input)
        : new DateTimeZone('UTC');

    $nowOtc  = new DateTimeImmutable('now', $timezone);
    $year = (int) $nowOtc->format('Y');

    $hour = $nowOtc->format('H');
    $min  = $nowOtc->format('i');
    $sec  = $nowOtc->format('s');

    $new_year = new DateTimeImmutable("{$year}-03-20", $timezone);
    if ($nowOtc < $new_year) {
        $year--;
        $new_year = new DateTimeImmutable("{$year}-03-20", $timezone);
    }

    $final_year = $year - 2000;
    $final_day  = (int) $new_year->diff($nowOtc)->days + 1;

    $zero_year      = str_pad((string) $final_year, 3, '0', STR_PAD_LEFT);
    $zero_final_day = str_pad((string) $final_day,  3, '0', STR_PAD_LEFT);

    // print date
    $o_date = ($hour . ":" . $min . ":" . $sec . " " . $final_day . " " . $zero_year);

    // file date
    $o_file_date  = $zero_year;
    $o_file_date .= $zero_final_day;
    $o_file_date .= $hour;
    $o_file_date .= $min;
    $o_file_date .= $sec;
?>