diff --git a/src/Type/Php/DateIntervalFormatDynamicReturnTypeExtension.php b/src/Type/Php/DateIntervalFormatDynamicReturnTypeExtension.php
new file mode 100644
index 0000000000..f6d32b02ed
--- /dev/null
+++ b/src/Type/Php/DateIntervalFormatDynamicReturnTypeExtension.php
@@ -0,0 +1,91 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Type\Php;
+
+use DateInterval;
+use PhpParser\Node\Expr\MethodCall;
+use PHPStan\Analyser\Scope;
+use PHPStan\DependencyInjection\AutowiredService;
+use PHPStan\Reflection\MethodReflection;
+use PHPStan\Type\Accessory\AccessoryLowercaseStringType;
+use PHPStan\Type\Accessory\AccessoryNonEmptyStringType;
+use PHPStan\Type\Accessory\AccessoryNonFalsyStringType;
+use PHPStan\Type\Accessory\AccessoryNumericStringType;
+use PHPStan\Type\Accessory\AccessoryUppercaseStringType;
+use PHPStan\Type\DynamicMethodReturnTypeExtension;
+use PHPStan\Type\IntersectionType;
+use PHPStan\Type\StringType;
+use PHPStan\Type\Type;
+use PHPStan\Type\TypeCombinator;
+use function count;
+use function is_numeric;
+use function strtolower;
+use function strtoupper;
+
+#[AutowiredService]
+final class DateIntervalFormatDynamicReturnTypeExtension implements DynamicMethodReturnTypeExtension
+{
+
+	public function getClass(): string
+	{
+		return DateInterval::class;
+	}
+
+	public function isMethodSupported(MethodReflection $methodReflection): bool
+	{
+		return $methodReflection->getName() === 'format';
+	}
+
+	public function getTypeFromMethodCall(MethodReflection $methodReflection, MethodCall $methodCall, Scope $scope): ?Type
+	{
+		$arguments = $methodCall->getArgs();
+
+		if (!isset($arguments[0])) {
+			return null;
+		}
+
+		$arg = $scope->getType($arguments[0]->value);
+
+		$constantStrings = $arg->getConstantStrings();
+		if (count($constantStrings) === 0) {
+			if ($arg->isNonEmptyString()->yes()) {
+				return new IntersectionType([new StringType(), new AccessoryNonEmptyStringType()]);
+			}
+
+			return null;
+		}
+
+		// The worst case scenario for the non-falsy-string check is that every number is 0.
+		$dateInterval = new DateInterval('P0D');
+
+		$possibleReturnTypes = [];
+		foreach ($constantStrings as $string) {
+			$value = $dateInterval->format($string->getValue());
+
+			$accessories = [];
+			if (is_numeric($value)) {
+				$accessories[] = new AccessoryNumericStringType();
+			}
+			if ($value !== '0' && $value !== '') {
+				$accessories[] = new AccessoryNonFalsyStringType();
+			} elseif ($value !== '') {
+				$accessories[] = new AccessoryNonEmptyStringType();
+			}
+			if (strtolower($value) === $value) {
+				$accessories[] = new AccessoryLowercaseStringType();
+			}
+			if (strtoupper($value) === $value) {
+				$accessories[] = new AccessoryUppercaseStringType();
+			}
+
+			if (count($accessories) === 0) {
+				return null;
+			}
+
+			$possibleReturnTypes[] = new IntersectionType([new StringType(), ...$accessories]);
+		}
+
+		return TypeCombinator::union(...$possibleReturnTypes);
+	}
+
+}
diff --git a/tests/PHPStan/Analyser/nsrt/date-interval-format.php b/tests/PHPStan/Analyser/nsrt/date-interval-format.php
new file mode 100644
index 0000000000..691d9e3328
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/date-interval-format.php
@@ -0,0 +1,40 @@
+<?php
+
+namespace DateIntervalFormat;
+
+use DateInterval;
+
+use function PHPStan\Testing\assertType;
+
+class Foo
+{
+	/**
+	 * @param string           $string
+	 * @param non-empty-string $nonEmptyString
+	 * @param '%Y'|'%D'        $unionString1
+	 * @param '%Y'|'%y'        $unionString2
+	 *
+	 * @return void
+	 */
+	public function test(
+		DateInterval $dateInterval,
+		string $string,
+		string $nonEmptyString,
+		string $unionString1,
+		string $unionString2,
+	): void {
+		assertType('string', $dateInterval->format($string));
+		assertType('non-empty-string', $dateInterval->format($nonEmptyString));
+
+		assertType('lowercase-string&non-falsy-string&numeric-string&uppercase-string', $dateInterval->format('%Y')); // '00'
+		assertType('lowercase-string&non-empty-string&numeric-string&uppercase-string', $dateInterval->format('%y')); // '0'
+		assertType('lowercase-string&non-falsy-string&numeric-string&uppercase-string', $dateInterval->format($unionString1));
+		assertType('lowercase-string&non-empty-string&numeric-string&uppercase-string', $dateInterval->format($unionString2));
+
+		assertType('non-falsy-string&uppercase-string', $dateInterval->format('%Y DAYS'));
+		assertType('non-falsy-string&uppercase-string', $dateInterval->format($unionString1. ' DAYS'));
+
+		assertType('lowercase-string&non-falsy-string', $dateInterval->format('%Y days'));
+		assertType('lowercase-string&non-falsy-string', $dateInterval->format($unionString1. ' days'));
+	}
+}