// Created on savesnippets.com · https://savesnippets.com/jUxDeaA8RVGydg
<?php
function toUserTz(int|string $utcTime, string $tz = 'America/Chicago', string $format = 'Y-m-d H:i T'): string {
    $t  = is_int($utcTime) ? $utcTime : strtotime($utcTime . ' UTC');
    $dt = new DateTime('@' . $t);          // always UTC
    $dt->setTimezone(new DateTimeZone($tz));
    return $dt->format($format);
}

echo toUserTz(1735689600, 'America/Chicago'); // 2024-12-31 18:00 CST
echo toUserTz(1735689600, 'Asia/Tokyo');      // 2025-01-01 09:00 JST