If you’ve ever spent hours debugging a date-related bug only to discover that some code somewhere modified a DateTime object you thought was safe, you know the pain. DateTime in PHP is mutable, which means any code that touches your date object can silently change it, leading to bugs that are incredibly challenging to track down.

The good news is PHP has given us a better way: DateTimeImmutable. Combined with proper timezone handling, custom wrapper classes, and DatePeriod for working with date ranges, you can write date and time code that’s both safer and more maintainable.

In this article, we’ll discuss why it’s easy to accidently create bugs with DateTime, how DateTimeImmutable solves the problem, how to create a custom AppDateTime wrapper to enforce immutability across your codebase, and practical uses of DateTimeZone and DatePeriod.

If you’re new here, we cover topics related to the PHP ecosystem. Hit subscribe so you don’t miss the next one.

The Problem with DateTime

Let me show you why DateTime is bug prone. Imagine you have this seemingly innocent code:

<?php
$recordingDate = new DateTime("2025-12-17 10:00:00");
$publishDate = calculatePublishDate($recordingDate);

echo "Episode recorded: " . $recordingDate->format("Y-m-d H:i:s") . "\n";
echo "Publish on: " . $publishDate->format("Y-m-d H:i:s") . "\n";

function calculatePublishDate(DateTime $date): DateTime
{
    $date->modify("+7 days");
    return $date;
}

You might think the recording date stays at December 17th and the publish date becomes December 24th. But here’s what actually happens:

Episode recorded: 2025-12-24 10:00:00
Publish on: 2025-12-24 10:00:00

Annoyingly, both of the dates are the same. The calculatePublishDate() function modified the original $recordingDate object because DateTime is mutable and PHP passes parameter by reference. Now your scheduling system thinks you recorded the episode on the publish date, breaking other pieces of your code. This is a sneaky bug that can easily slip through code reviews and cause problems in production.

DateTimeImmutable to the Rescue

The DateTimeImmutable class solves this by being immutable which means it always returns a new instance every time you modify it. Here’s the same podcast scheduling code using DateTimeImmutable:

<?php
$recordingDate = new DateTimeImmutable("2025-12-17 10:00:00");
$publishDate = calculatePublishDate($recordingDate);

echo "Episode recorded: " . $recordingDate->format("Y-m-d H:i:s") . "\n";
echo "Publish on: " . $publishDate->format("Y-m-d H:i:s") . "\n";

function calculatePublishDate(DateTimeImmutable $date): DateTimeImmutable
{
    return $date->modify("+7 days");
}

Now we get the expected output:

Episode recorded: 2025-12-17 10:00:00
Publish on: 2025-12-24 10:00:00

The $recordingDate variable is unchanged because the DateTimeImmutable version of modify() returns a brand new DateTimeImmutable instance. If you forget to capture the return value, your code won’t work, and you’ll catch the bug immediately instead of hours later.

Working with Timezones

One of the most common sources of date bugs is timezone confusion. Timezones are confusing at the best of times and down right baffling at the worst because we need to handle things like inconsistent Daylight Saving Time rules and spots where timezones have offsets that aren’t full hour increments.

The DateTimeZone class makes converting from one timezone to another easy:

<?php
// Always store past datetimes in UTC
// because it doesn't have DST rules
$episodePublishTime = new DateTimeImmutable(
    "2025-12-17 15:00:00",
    new DateTimeZone("UTC")
);

// Convert to listener's timezone for display
$listenerTimezone = new DateTimeZone("America/New_York");
$localTime = $episodePublishTime
    ->setTimezone($listenerTimezone);

echo "Stored in DB (UTC): " . $episodePublishTime->format("Y-m-d H:i:s T") . "\n";
echo "View for listener (EST): " . $localTime->format("Y-m-d H:i:s T") . "\n";

Output:

Stored in DB (UTC): 2025-12-17 15:00:00 UTC
View for listener (EST): 2025-12-17 10:00:00 EST

A common gotcha is forgetting to specify a timezone when creating dates. PHP will use the default timezone as it’s specified in your “php.ini”, which might not be what you expect. Because of this it’s a good idea to always specify the timezone as you never know when someone’s going to change a setting and not realize what it will do.

Using DatePeriod for Date Ranges

The DatePeriod class is incredibly powerful when you need to work with sequences of dates and times. Common use cases include generating calendars or scheduling recurring tasks.

Here’s how to generate a weekly podcast publishing schedule:

<?php
$firstEpisode = new DateTimeImmutable("2025-12-01");
$lastEpisode = new DateTimeImmutable("2025-12-29");
$weeklyInterval = new DateInterval("P7D"); // 7 days (weekly)

$publishSchedule = new DatePeriod($firstEpisode, $weeklyInterval, $lastEpisode);

foreach ($publishSchedule as $publishDate) {
    echo "Publish episode on: " . $publishDate->format("Y-m-d") . "\n";
}

Output:

Publish episode on: 2025-12-01
Publish episode on: 2025-12-08
Publish episode on: 2025-12-15
Publish episode on: 2025-12-22

Notice that DatePeriod excludes the end date by default. If you want to include December 29th in your schedule, you need to add one interval to your end date:

<?php
// Include the final episode on Dec 29th
$lastEpisode = $lastEpisode->add($weeklyInterval);

You can also use DatePeriod with recurrences instead of an end date. This is perfect for planning out a season of podcast episodes:

<?php
$seasonStart = new DateTimeImmutable("2025-01-06");
$weeklyInterval = new DateInterval("P7D"); // Weekly episodes
$numberOfEpisodes = 12; // 12-episode season

$episodeSchedule = new DatePeriod($seasonStart, $weeklyInterval, $numberOfEpisodes);

foreach ($episodeSchedule as $episodeDate) {
    echo "Episode publishes: " . $episodeDate->format("Y-m-d") . "\n";
}

When you construct a DateInterval it uses a special format string. It’s based on the ISO 8601 duration specification and can be confusing if you’re uninitated. As some examples, “P1D” means “Period of 1 Day”, “P1M” means “Period of 1 Month”, “PT1H” means “Period Time 1 Hour”. The “T” separates date parts from time parts.

Here are common intervals:
– P1D: 1 day
– P7D: 1 week (7 days)
– P1M: 1 month
– P1Y: 1 year
– PT1H: 1 hour
– PT30M: 30 minutes
– P1DT12H: 1 day and 12 hours

Creating an AppDateTime Class

While DateTimeImmutable is a wonderful additional to your toolbox, you can make your codebase more powerful by creating a custom DateTimeImmutable class that extends the DateTimeImmutable. This makes it easier to notice when someone accidentally uses DateTime, add helpful methods, and provide better error messages.

Here’s a basic AppDateTime class:

<?php

declare(strict_types=1);

namespace App\Types\DateTime;

use DateTimeImmutable;

final class AppDateTime extends DateTimeImmutable
{
    /**
     * Format the datetime as ISO 8601 string.
     */
    public function toIso8601(): string
    {
        return $this->format("c");
    }

    /**
     * Format the datetime as a date string (Y-m-d).
     */
    public function toDateString(): string
    {
        return $this->format("Y-m-d");
    }

    /**
     * Convert the datetime to a string representation.
     */
    public function __toString(): string
    {
        return $this->format("Y-m-d H:i:s");
    }
}

Because we’re extending the DateTimeImmutable class it operates just like the global DateTimeImmutable but we can easily find places where someone is using the wrong DateTime class using automated tests.

What You Need To Know

1. Use DateTimeImmutable instead of DateTime as immutability reduces bugs.
2. Use DateTimeZone to manage timezone conversions.
3. Use DatePeriod for generating date ranges and recurring events.
4. DateInterval uses the ISO 8601 duration format (P1D, P1M, PT1H, etc).
5. Create an AppDateTime wrapper to enforce immutability and add convenience methods.