PHP Date and Time

PHP Date and Time: date(), DateTimeImmutable, and Timezones

PHP date and time functions let you format timestamps, calculate expiration windows, and convert dates across user timezones. While date() and strtotime() work for quick formatting, modern PHP applications rely on DateTimeImmutable and DateTimeZone for safe date math.

Formatting Dates with date() vs DateTimeImmutable

Always set a default timezone (usually 'UTC') in your bootstrap code or php.ini so server timestamps stay consistent across deployments. Store dates in UTC in your database and convert to the user's local timezone only when displaying them.

<?php
declare(strict_types=1);

$utcZone = new DateTimeZone('UTC');
$dhakaZone = new DateTimeZone('Asia/Dhaka');

$createdAt = new DateTimeImmutable('now', $utcZone);
$expiresAt = $createdAt->modify('+7 days');
$localView = $createdAt->setTimezone($dhakaZone);

echo "Stored (UTC): " . $createdAt->format('Y-m-d H:i:s') . PHP_EOL;
echo "Expires (UTC): " . $expiresAt->format('Y-m-d H:i:s') . PHP_EOL;
echo "Dhaka Display: " . $localView->format('M j, Y g:i A') . PHP_EOL;

Because DateTimeImmutable returns a new instance whenever you call modify() or setTimezone(), $createdAt never changes accidentally mid-function.

Parsing and Comparing Dates Safely

When parsing dates from APIs or forms, use DateTimeImmutable::createFromFormat() so ambiguous inputs like 04/05/2026 are never misread as month-first versus day-first.

<?php
declare(strict_types=1);

$invoiceDate = DateTimeImmutable::createFromFormat('!Y-m-d', '2026-11-01', new DateTimeZone('UTC'));
$dueDate = DateTimeImmutable::createFromFormat('!Y-m-d', '2026-11-15', new DateTimeZone('UTC'));

if ($invoiceDate && $dueDate) {
    $diff = $invoiceDate->diff($dueDate);
    echo "Payment window: {$diff->days} days" . PHP_EOL;
}

Common Mistakes to Avoid

• Using mutable DateTime when calculating intervals: Calling $date->modify('+1 day') on a mutable DateTime alters the original variable everywhere it is referenced.
• Omitting the ! prefix in createFromFormat(): Without ! (as in '!Y-m-d'), PHP fills the time portion with the current clock time instead of 00:00:00.
• Adding 86400 seconds to advance one calendar day: Daylight Saving Time transitions have 23-hour or 25-hour days. Use ->modify('+1 day') instead.

Frequently Asked Questions

What is the difference between DateTime and DateTimeImmutable?

DateTime mutates the object in place on every modification, while DateTimeImmutable leaves the original intact and returns a modified copy.

How do I get a Unix timestamp in PHP?

Call time() for the current Unix timestamp in seconds, or $dt->getTimestamp() on any DateTimeInterface object.

Why does date('Y-m-d') sometimes show yesterday's date?

Your PHP runtime timezone (date_default_timezone_get()) likely defaults to UTC while your local clock is ahead or behind UTC.