diff --git a/lib/Service/Appointments/AvailabilityGenerator.php b/lib/Service/Appointments/AvailabilityGenerator.php
index 5eba70200..fdf0b5616 100644
--- a/lib/Service/Appointments/AvailabilityGenerator.php
+++ b/lib/Service/Appointments/AvailabilityGenerator.php
@@ -70,8 +70,8 @@ class AvailabilityGenerator {
 		//      when the user opens the page at 10:17.
 		// But only do this when the time isn't already a "pretty" time
 		if ($earliestStart % $config->getIncrement() !== 0) {
-			$roundTo = (int)round(($config->getIncrement()) / 300) * 300;
-			$earliestStart = (int)ceil($earliestStart / $roundTo) * $roundTo;
+			$roundTo = (int)(round(($config->getIncrement()) / 300) * 300);
+			$earliestStart = (int)(ceil($earliestStart / $roundTo) * $roundTo);
 		}
 
 		$latestEnd = min(
diff --git a/tests/php/unit/Service/Appointments/AvailabilityGeneratorTest.php b/tests/php/unit/Service/Appointments/AvailabilityGeneratorTest.php
index 33def004f..8664e5b08 100644
--- a/tests/php/unit/Service/Appointments/AvailabilityGeneratorTest.php
+++ b/tests/php/unit/Service/Appointments/AvailabilityGeneratorTest.php
@@ -119,6 +119,48 @@ class AvailabilityGeneratorTest extends TestCase {
 		);
 	}
 
+	public function testNoAvailabilityQuarterHourIncrementsFourtyFiveMinutesLenght(): void {
+		$config = new AppointmentConfig();
+		$config->setLength(2700); // 45 minutes
+		$config->setIncrement(900); // 15 minutes
+		$config->setAvailability(null);
+
+		$this->timeFactory->expects(self::once())
+			->method('getTime')
+			->willReturn(1709546400);
+
+		$sdate = new \DateTime('2024-03-04 10:00');
+		$edate = new \DateTime('2024-03-04 10:45');
+
+		$slots = $this->generator->generate($config, $sdate->getTimestamp(), $edate->getTimestamp());
+
+		self::assertCount(1, $slots);
+		self::assertEquals(0, ($slots[0]->getStart() % $config->getIncrement()));
+		self::assertEquals(45 * 60, ($slots[0]->getEnd() - $slots[0]->getStart()));
+		self::assertEquals(
+			[new Interval(1709546400, 1709549100)],
+			$slots,
+		);
+	}
+
+	public function testNoAvailabilityZeroTo2700QuarterHourIncrementsFourtyFiveMinutesLenght(): void {
+		$config = new AppointmentConfig();
+		$config->setLength(2700); // 45 minutes
+		$config->setIncrement(900); // 15 minutes
+		$config->setAvailability(null);
+
+		$slots = $this->generator->generate($config, 0, 2700);
+
+		self::assertCount(1, $slots);
+		self::assertEquals(0, $slots[0]->getStart() % 2700);
+		self::assertEquals(45 * 60, ($slots[0]->getEnd() - $slots[0]->getStart()));
+		self::assertEquals(
+			[new Interval(0, 45 * 60)],
+			$slots,
+		);
+	}
+
+
 	public function testNoAvailabilitySetRoundWithRealLifeTimesUgly(): void {
 		$config = new AppointmentConfig();
 		$config->setLength(900);