Dates, Time Zones and Immutability

Use DateTimeImmutable (not DateTime) so operations return new objects instead of mutating shared instances. Store timestamps in UTC, convert to the user's time zone only for display, and set date.timezone explicitly. Use DateInterval/DatePeriod for ranges, and an injectable clock (PSR-20 ClockInterface) to make time-dependent code testable.

<?php
$utc = new DateTimeZone('UTC');
$createdAt = new DateTimeImmutable('now', $utc);
$expiresAt = $createdAt->modify('+30 minutes');       // new object

$local = $createdAt->setTimezone(new DateTimeZone('Europe/Berlin'));
echo $local->format('Y-m-d H:i T');

final class TokenService
{
    public function __construct(private Psr\Clock\ClockInterface $clock) {}

    public function isExpired(DateTimeImmutable $expiresAt): bool
    {
        return $expiresAt <= $this->clock->now();
    }
}

Common pitfalls

• Mutating a shared DateTime passed into a function.
• Storing local times without a time zone.
• Adding '+1 month' to Jan 31 and expecting Feb 28 (it overflows into March).

Key points

• Prefer DateTimeImmutable
• Store UTC, display local
• Inject a clock for testability

Further reading: PHP manual: DateTimeImmutable