diff --git a/CHANGELOG.md b/CHANGELOG.md
index 73cd9e9..34230f7 100755
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,12 @@
 
 ## [Unreleased]
 
+## [2.2.1] - 2020-12-09
+
+### Fixed
+
+- Fix support for `DateTimeImmutable` [#90](https://github.com/rlanvin/php-rrule/issues/90)
+
 ## [2.2.0] - 2019-11-01
 
 ### Added
@@ -190,7 +196,8 @@
 
 - First release, everything before that was unversioned (`dev-master` was used).
 
-[Unreleased]: https://github.com/rlanvin/php-rrule/compare/v2.2.0...HEAD
+[Unreleased]: https://github.com/rlanvin/php-rrule/compare/v2.2.1...HEAD
+[2.2.1]: https://github.com/rlanvin/php-rrule/compare/v2.2.0...v2.2.1
 [2.2.0]: https://github.com/rlanvin/php-rrule/compare/v2.1.0...v2.2.0
 [2.1.0]: https://github.com/rlanvin/php-rrule/compare/v2.0.0...v2.1.0
 [2.0.0]: https://github.com/rlanvin/php-rrule/compare/v2.0.0-rc1...v2.0.0
diff --git a/src/RRule.php b/src/RRule.php
index 5f867b7..3edda37 100755
--- a/src/RRule.php
+++ b/src/RRule.php
@@ -1350,10 +1350,10 @@ class RRule implements RRuleInterface
 			$dtstart = clone $occurrence; // since DateTime is not immutable, clone to avoid any problem
 			// so we skip the last occurrence of the cache
 			if ($this->freq === self::SECONDLY) {
-				$dtstart->modify('+'.$this->interval.'second');
+				$dtstart = $dtstart->modify('+'.$this->interval.'second');
 			}
 			else {
-				$dtstart->modify('+1second');
+				$dtstart = $dtstart->modify('+1second');
 			}
 		}
 
@@ -1367,7 +1367,7 @@ class RRule implements RRuleInterface
 			// calculation magic at the end of the loop (when incrementing)
 			// to realign on first pass.
 			$tmp = clone $dtstart;
-			$tmp->modify('-'.pymod($dtstart->format('N') - $this->wkst,7).'days');
+			$tmp = $tmp->modify('-'.pymod($dtstart->format('N') - $this->wkst,7).'days');
 			list($year,$month,$day,$hour,$minute,$second) = explode(' ',$tmp->format('Y n j G i s'));
 			unset($tmp);
 		}
diff --git a/src/RRuleTrait.php b/src/RRuleTrait.php
index a68c6bc..769a8a8 100755
--- a/src/RRuleTrait.php
+++ b/src/RRuleTrait.php
@@ -173,7 +173,7 @@ trait RRuleTrait
 	 * Convert any date into a DateTime object.
 	 *
 	 * @param mixed $date
-	 * @return \DateTime
+	 * @return \DateTimeInterface Returns a DateTimeImmutable if a DateTimeImmutable is passed, or DateTime otherwise
 	 *
 	 * @throws \InvalidArgumentException on error
 	 */
diff --git a/tests/RRuleTest.php b/tests/RRuleTest.php
index 2c28583..f3cfe15 100755
--- a/tests/RRuleTest.php
+++ b/tests/RRuleTest.php
@@ -2779,6 +2779,41 @@ class RRuleTest extends TestCase
 		$this->assertTrue($rule['DTSTART'] instanceof \DateTime);
 	}
 
+	/**
+	 * Test Bug #90
+	 * @see https://github.com/rlanvin/php-rrule/issues/90
+	 */
+	public function testDateImmutable()
+	{
+		$dtstart_immutable = \DateTimeImmutable::createFromFormat('Y-m-d H:i', '2021-01-08 08:00');
+		//$dtstart_mutable = \DateTime::createFromFormat('Y-m-d H:i', '2021-01-08 08:00');
+
+		$rrule = new RRule([
+			'BYDAY' => ['MO', 'WE', 'FR'],
+			'FREQ' => 'WEEKLY',
+			'WKST' => 'SU',
+			'DTSTART' => $dtstart_immutable,
+		]);
+
+		$start = \DateTimeImmutable::createFromFormat('Y-m-d', '2020-01-01');
+		$end = \DateTimeImmutable::createFromFormat('Y-m-d', '2021-12-31');
+
+		$occurrences = $rrule->getOccurrencesBetween($start, $end, 10);
+
+		$this->assertEquals([
+			new DateTime('Friday, January 8, 2021 08:00'),
+			new DateTime('Monday, January 11, 2021 08:00'),
+			new DateTime('Wednesday, January 13, 2021 08:00'),
+			new DateTime('Friday, January 15, 2021 08:00'),
+			new DateTime('Monday, January 18, 2021 08:00'),
+			new DateTime('Wednesday, January 20, 2021 08:00'),
+			new DateTime('Friday, January 22, 2021 08:00'),
+			new DateTime('Monday, January 25, 2021 08:00'),
+			new DateTime('Wednesday, January 27, 2021 08:00'),
+			new DateTime('Friday, January 29, 2021 08:00')
+		], $occurrences, 'DateTimeImmutable produces valid results');
+	}
+
 ///////////////////////////////////////////////////////////////////////////////
 // Array access and countable interfaces