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How do you format a date or time value in PHP using the format method, and what do common format characters like Y, m, and d represent? How do you add or subtract a specific amount of time from a date using PHP's DateInterval and the modify or add methods? How does PHP's DateTime class handle time zones, and why is explicitly setting a time zone important for applications serving users in different regions? How do you calculate the difference between two dates in PHP, and what information does the DateInterval object returned by the diff method provide? How should an application handle daylight saving time transitions correctly when performing date arithmetic across such a transition?
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What is the difference between DateTime and DateTimeImmutable, and why might a codebase deliberately prefer the immutable version despite the extra verbosity?

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Answer

DateTime is mutable, meaning methods like add or modify change the object in place and return the same modified instance, which can lead to subtle bugs if a date object is accidentally shared and modified in one part of the code while another part expected it to remain unchanged, while DateTimeImmutable methods instead return a brand new DateTimeImmutable instance representing the modified date, leaving the original completely untouched, which many teams prefer specifically because it eliminates an entire category of unintended side effect bugs.
$original = new DateTimeImmutable('2026-01-01');
$modified = $original->add(new DateInterval('P1M'));
echo $original->format('Y-m-d');
echo $modified->format('Y-m-d');
Real-world example A team refactors a scheduling system after tracing a subtle bug back to a shared DateTime object being unexpectedly modified by one function while another function still expected its original value, switching to DateTimeImmutable to prevent this entire class of bug from recurring.

Common follow-ups: How do you migrate an existing codebase from DateTime to DateTimeImmutable safely?;Is there a meaningful performance difference between DateTime and DateTimeImmutable?

OOP;Design Patterns in PHP
All 7 Date & Time Handling questions