DateTime Class Since: PHP 5.3（2009）

// Creates a DateTime object for the current or a specified date/time.
$dt = new DateTime($datetime, $timezone);

// Formats the date/time and returns it as a string.
$dt->format($format);

// Modifies the date/time using a relative date/time expression.
$dt->modify($modifier);

// Adds a DateInterval to the date/time.
$dt->add($interval);

// Subtracts a DateInterval from the date/time.
$dt->sub($interval);

// Returns the difference between two date/time values as a DateInterval.
$dt->diff($targetDateTime);

format() returns the formatted date/time as a string. modify(), add(), and sub() return the object itself, enabling method chaining. diff() returns a DateInterval object.

<?php
// Creates a DateTime object for the current date and time.
$now = new DateTime();
echo $now->format("Y-m-d H:i:s"); // Outputs the current date and time.

// Creates a DateTime object from a string.
$dt = new DateTime("2025-04-15 10:30:00");
echo $dt->format("Y-m-d H:i"); // Outputs "2025-04-15 10:30".

// Changes the date/time using modify().
$dt->modify("+1 month");
echo $dt->format("Y-m-d"); // Outputs "2025-05-15", one month later.

// Chains multiple operations together.
$result = (new DateTime("2025-01-01"))->modify("+6 months")->format("Y-m-d");
echo $result; // Outputs "2025-07-01".

// Adds and subtracts time using add() and sub() with DateInterval.
$dt2 = new DateTime("2025-04-15");
$dt2->add(new DateInterval("P10D")); // Adds 10 days.
echo $dt2->format("Y-m-d"); // Outputs "2025-04-25".

$dt2->sub(new DateInterval("P2M")); // Subtracts 2 months.
echo $dt2->format("Y-m-d"); // Outputs "2025-02-25".

// Calculates the difference between two dates using diff().
$start = new DateTime("2025-01-01");
$end = new DateTime("2025-04-15");
$interval = $start->diff($end);
echo $interval->days . " days"; // Outputs "104 days".
echo $interval->format("%m months and %d days"); // Outputs "3 months and 14 days".

// Practical example: calculating a person's age.
$birthday = new DateTime("1990-08-20");
$today = new DateTime();
$age = $birthday->diff($today);
echo $age->y . " years old"; // Outputs the current age.

// Creates DateTime objects with specific timezones.
$tokyo = new DateTime("now", new DateTimeZone("Asia/Tokyo"));
$utc = new DateTime("now", new DateTimeZone("UTC"));
echo $tokyo->format("H:i") . " / " . $utc->format("H:i"); // Compares Japan time and UTC.

// Using DateTimeImmutable prevents the original object from being modified.
$original = new DateTimeImmutable("2025-04-15");
$modified = $original->modify("+1 day"); // Returns a new object.
echo $original->format("Y-m-d"); // The original remains "2025-04-15".
echo $modified->format("Y-m-d"); // The modified version is "2025-04-16".

The DateTime class is an object-oriented API for date and time manipulation introduced in PHP 5.3. In addition to the same functionality as date() and strtotime(), it also supports advanced features such as timezone management and difference calculations.

DateTime objects are mutable, so calling modify(), add(), or sub() changes the object in place. To avoid unintended side effects, use DateTimeImmutable instead. DateTimeImmutable returns a new object on every method call, leaving the original unchanged.

The DateInterval format follows the ISO 8601 duration notation. For example, "P1Y2M3D" means 1 year, 2 months, and 3 days; "PT1H30M" means 1 hour and 30 minutes. When specifying a time component after P, you must insert a T before the time portion.