Return ChronosInterval from Chronos::diff()

docs/.vitepress/toc_en.json

@@ -5,6 +5,7 @@
       "collapsed": false,
       "items": [
         { "text": "Introduction", "link": "/index" },
+        { "text": "4.x Migration Guide", "link": "/4-x-migration-guide" },
         { "text": "3.x Migration Guide", "link": "/3-x-migration-guide" },
         { "text": "API", "link": "https://api.cakephp.org/chronos" }
       ]

docs/en/4-x-migration-guide.md

@@ -0,0 +1,28 @@
+# 4.x Migration Guide
+
+Chronos 4.x contains breaking changes that could impact your application. This
+guide provides an overview of the breaking changes made in 4.x
+
+## diff() and fromNow() return ChronosInterval
+
+`Chronos::diff()`, `ChronosDate::diff()` and `Chronos::fromNow()` now return a
+`ChronosInterval` instead of a `DateInterval`. `Chronos::fromNow()` previously
+returned `DateInterval|false`; it now always returns a `ChronosInterval`.
+
+`ChronosInterval` decorates the native `DateInterval` and exposes the same
+properties (`y`, `m`, `d`, `h`, `i`, `s`, `f`, `invert`, `days`), so most code
+keeps working unchanged. However, it does **not** extend `DateInterval`: code
+that type-hints `DateInterval` or relies on `instanceof DateInterval` against
+the result must call `->toNative()` to get the underlying `DateInterval`:
+
+```php
+// Before (3.x)
+$interval = $first->diff($second); // DateInterval
+
+// After (4.x), when a native DateInterval is required
+$interval = $first->diff($second)->toNative(); // DateInterval
+```
+
+In return, the result gains ISO 8601 duration formatting and a number of
+convenience methods. See [Working with Intervals](/index#working-with-intervals)
+for the full overview.

docs/en/index.md

@@ -210,7 +210,7 @@ $time->isWithinNext('3 hours');
 In addition to comparing datetimes, calculating differences or deltas between
 two values is a common task:
 ```php
-// Get a DateInterval representing the difference
+// Get a ChronosInterval representing the difference
 $first->diff($second);
 
 // Get difference as a count of specific units.
@@ -230,6 +230,59 @@ echo $date->diffForHumans();
 echo $date->diffForHumans($other); // 1 hour ago;
 ```
 
+## Working with Intervals
+
+> [!TIP] Changed in 4.x
+> `diff()` and `fromNow()` previously returned a native `DateInterval`. See the
+> [4.x Migration Guide](/4-x-migration-guide).
+
+`Chronos::diff()`, `ChronosDate::diff()` and `Chronos::fromNow()` return a
+`ChronosInterval`. It decorates the native `DateInterval`, so all the usual
+properties (`y`, `m`, `d`, `h`, `i`, `s`, `f`, `invert`, `days`) keep working
+while adding convenience methods on top:
+```php
+$interval = $first->diff($second);
+
+// ISO 8601 duration string. __toString() returns the same value.
+echo $interval->toIso8601String(); // P1Y2M3D
+echo $interval;                    // P1Y2M3D
+
+// Totals. totalDays() is exact when the interval comes from diff();
+// totalSeconds() approximates using 30-day months and 365-day years.
+$interval->totalDays();
+$interval->totalSeconds();
+
+// State checks.
+$interval->isZero();
+$interval->isNegative();
+
+// A strtotime()-compatible relative string.
+echo $interval->toDateString(); // 1 year 2 months 3 days
+
+// Component-wise arithmetic (no overflow normalization).
+$interval->add($other);
+$interval->sub($other);
+```
+You can also build intervals directly:
+```php
+use Cake\Chronos\ChronosInterval;
+
+ChronosInterval::create('P1Y2M3D');
+ChronosInterval::createFromValues(years: 1, months: 2, days: 3);
+ChronosInterval::createFromDateString('1 year 2 days');
+ChronosInterval::instance($dateInterval);
+```
+When an API requires a native `DateInterval`, call `toNative()`:
+```php
+$native = $first->diff($second)->toNative();
+```
+
+> [!NOTE]
+> `ChronosInterval` is a decorator and does **not** extend `DateInterval`, so
+> code that type-hints `DateInterval` or relies on `instanceof DateInterval`
+> against the result of `diff()`/`fromNow()` must call `->toNative()` to get
+> the wrapped `DateInterval` back.
+
 ## Formatting Strings
 
 Chronos provides a number of methods for displaying our outputting datetime

phpstan.neon

@@ -17,3 +17,6 @@ parameters:
 			message: "#with generic class DatePeriod but does not specify its types: TDate, TEnd, TRecurrences$#"
 			count: 1
 			path: src/ChronosDatePeriod.php
+		-
+			identifier: method.childReturnType
+			path: src/Chronos.php

src/Chronos.php

@@ -20,6 +20,7 @@
 use DateTimeInterface;
 use DateTimeZone;
 use InvalidArgumentException;
+use ReturnTypeWillChange;
 use RuntimeException;
 use Stringable;
 
@@ -1011,11 +1012,12 @@ public function modify(string $modifier): static
      *
      * @param \DateTimeInterface $target Target instance
      * @param bool $absolute Whether the interval is forced to be positive
-     * @return \DateInterval
+     * @return \Cake\Chronos\ChronosInterval
      */
-    public function diff(DateTimeInterface $target, bool $absolute = false): DateInterval
+    #[ReturnTypeWillChange]
+    public function diff(DateTimeInterface $target, bool $absolute = false): ChronosInterval
     {
-        return parent::diff($target, $absolute);
+        return new ChronosInterval(parent::diff($target, $absolute));
     }
 
     /**
@@ -2778,9 +2780,9 @@ public function secondsUntilEndOfDay(): int
      * Convenience method for getting the remaining time from a given time.
      *
      * @param \DateTimeInterface $other The date to get the remaining time from.
-     * @return \DateInterval|bool The DateInterval object representing the difference between the two dates or FALSE on failure.
+     * @return \Cake\Chronos\ChronosInterval The ChronosInterval object representing the difference between the two dates.
      */
-    public static function fromNow(DateTimeInterface $other): DateInterval|bool
+    public static function fromNow(DateTimeInterface $other): ChronosInterval
     {
         $timeNow = new static();
 

src/ChronosDate.php

@@ -407,11 +407,11 @@ public function setISODate(int $year, int $week, int $dayOfWeek = 1): static
      *
      * @param \Cake\Chronos\ChronosDate $target Target instance
      * @param bool $absolute Whether the interval is forced to be positive
-     * @return \DateInterval
+     * @return \Cake\Chronos\ChronosInterval
      */
-    public function diff(ChronosDate $target, bool $absolute = false): DateInterval
+    public function diff(ChronosDate $target, bool $absolute = false): ChronosInterval
     {
-        return $this->native->diff($target->native, $absolute);
+        return new ChronosInterval($this->native->diff($target->native, $absolute));
     }
 
     /**

tests/TestCase/ChronosIntervalTest.php

@@ -15,11 +15,32 @@
 namespace Cake\Chronos\Test\TestCase;
 
 use Cake\Chronos\Chronos;
+use Cake\Chronos\ChronosDate;
 use Cake\Chronos\ChronosInterval;
 use DateInterval;
 
 class ChronosIntervalTest extends TestCase
 {
+    public function testChronosDiffReturnsChronosInterval(): void
+    {
+        $start = new Chronos('2020-01-01');
+        $end = new Chronos('2020-01-11');
+        $diff = $start->diff($end);
+
+        $this->assertInstanceOf(ChronosInterval::class, $diff);
+        $this->assertSame(10, $diff->d);
+    }
+
+    public function testChronosDateDiffReturnsChronosInterval(): void
+    {
+        $start = ChronosDate::create(2020, 1, 1);
+        $end = ChronosDate::create(2020, 1, 11);
+        $diff = $start->diff($end);
+
+        $this->assertInstanceOf(ChronosInterval::class, $diff);
+        $this->assertSame(10, $diff->d);
+    }
+
     public function testCreateFromSpec(): void
     {
         $interval = ChronosInterval::create('P1Y2M3D');
@@ -88,10 +109,8 @@ public function testToIso8601StringNegative(): void
     {
         $past = new Chronos('2020-01-01');
         $future = new Chronos('2021-02-02');
-        $diff = $past->diff($future);
-        $diff->invert = 1;
+        $interval = $future->diff($past);
 
-        $interval = ChronosInterval::instance($diff);
         $this->assertStringStartsWith('-P', $interval->toIso8601String());
     }
 
@@ -123,9 +142,8 @@ public function testTotalDaysFromDiff(): void
     {
         $start = new Chronos('2020-01-01');
         $end = new Chronos('2020-01-11');
-        $diff = $start->diff($end);
+        $interval = $start->diff($end);
 
-        $interval = ChronosInterval::instance($diff);
         $this->assertSame(10, $interval->totalDays());
     }
 
@@ -136,9 +154,8 @@ public function testIsNegative(): void
 
         $past = new Chronos('2020-01-01');
         $future = new Chronos('2020-01-02');
-        $diff = $future->diff($past);
+        $interval = $future->diff($past);
 
-        $interval = ChronosInterval::instance($diff);
         $this->assertTrue($interval->isNegative());
     }
 
@@ -296,9 +313,8 @@ public function testToDateStringNegative(): void
     {
         $past = new Chronos('2020-01-01');
         $future = new Chronos('2020-01-02');
-        $diff = $future->diff($past);
+        $interval = $future->diff($past);
 
-        $interval = ChronosInterval::instance($diff);
         $this->assertStringStartsWith('-', $interval->toDateString());
     }