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en/articles/from-tg/series/php-can-do/datetime-immutable.md
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# DateTimeImmutable
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*When time changes but objects don't*
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- **Added in:** PHP 5.5
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- **What it gives you:** a class that behaves like `DateTime`, but doesn't change when modified β it returns a cloned object instead.
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## What it's about
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In PHP, if we change the time of a `DateTime` object, it changes everywhere it has been passed, because objects in PHP are always passed by reference, and this can lead to unexpected behaviour.
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`DateTimeImmutable` solves this problem β any operation on a date returns a new instance instead of mutating the old one.
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## When it really makes sense
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- in DTOs / Value Objects β so the time is guaranteed not to change after being passed along;
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- in schedulers, timetables, reports β when you compute many dates from a base one;
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- in business logic β when one date is derived from another (start/end of a period, a deadline, a promotion, etc.).
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## A problem that's easy to miss
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In production code, `DateTime` is often passed between services and DTOs, or cached in containers.
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And here comes the typical story:
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```php
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class Campaign {
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public function __construct(
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public string $name,
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public DateTime $startsAt,
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public DateTime $endsAt,
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) {}
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}
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$base = new DateTime('2025-10-01');
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$campaign = new Campaign(
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name: 'Autumn Sale',
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startsAt: $base,
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endsAt: $base->modify('+7 days')
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);
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echo $campaign->startsAt->format('Y-m-d'); // 2025-10-08 β
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echo $campaign->endsAt->format('Y-m-d'); // 2025-10-08
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```
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We expected the start to be October 1, but `$base` was mutated by `modify`, and as a result the start and the end are the same.
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The same code with `DateTimeImmutable` works correctly:
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```php
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$base = new DateTimeImmutable('2025-10-01');
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$campaign = new Campaign(
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name: 'Autumn Sale',
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startsAt: $base,
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endsAt: $base->modify('+7 days')
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);
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echo $campaign->startsAt->format('Y-m-d'); // 2025-10-01 β
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echo $campaign->endsAt->format('Y-m-d'); // 2025-10-08
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```
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## One more example
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```php
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class Period {
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public function __construct(
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public DateTimeImmutable $from,
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public DateTimeImmutable $to
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) {}
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public function days(): int {
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return $this->to->diff($this->from)->days;
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}
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}
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$period = new Period(
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from: $from = new DateTimeImmutable('2025-06-01'),
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to: $from->modify('+10 days'),
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);
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echo $period->days(); // 10
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```
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No unexpected mutations β the object can be safely cached or passed further.
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## Conclusion
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> DateTimeImmutable is the safe choice for time-related logic.
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> It rules out accidental date changes, where one variable leads to unexpected results.
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If your date can change, sooner or later it will break something in a report, in the logic or in the finances. `Immutable` won't let that happen.
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β [Previous: Generators](/articles/from-tg/series/php-can-do/generators) Β· [Whole series](/articles/from-tg/series/php-can-do) Β· [Next: Named arguments β](/articles/from-tg/series/php-can-do/named-arguments)
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---
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*First published in Ukrainian in the Telegram channel [βCreated to dieβ](https://t.me/php_while_true/81) on October 21, 2025.*