diff --git a/src/PeriodTraits/PeriodGetters.php b/src/PeriodTraits/PeriodGetters.php
index 7ec6a7e..4c3808f 100644
--- a/src/PeriodTraits/PeriodGetters.php
+++ b/src/PeriodTraits/PeriodGetters.php
@@ -66,6 +66,13 @@ trait PeriodGetters
 
     public function length(): int
     {
+        // Excluding the boundaries of a period that is only a few units long can push the
+        // included start past the included end. Such a period contains nothing, which is what
+        // iterating over it yields, so its length is 0 rather than a distance measured backwards.
+        if ($this->includedStart() > $this->includedEnd()) {
+            return 0;
+        }
+
         // Length of month and year are not fixed, so we can't predict the length without iterate
         // TODO: maybe we can use cal_days_in_month ?
         if ($this->precision->equals(Precision::MONTH(), Precision::YEAR())) {
@@ -73,7 +80,7 @@ trait PeriodGetters
         }
 
         if ($this->precision->equals(Precision::HOUR(), Precision::MINUTE(), Precision::SECOND())) {
-            $length = abs($this->includedEnd()->getTimestamp() - $this->includedStart()->getTimestamp());
+            $length = $this->includedEnd()->getTimestamp() - $this->includedStart()->getTimestamp();
 
             if ($this->precision->equals(Precision::SECOND())) {
                 return $length + 1;
diff --git a/tests/BoundaryTest.php b/tests/BoundaryTest.php
index 2160dfd..d55889d 100644
--- a/tests/BoundaryTest.php
+++ b/tests/BoundaryTest.php
@@ -35,6 +35,11 @@ it('length with boundaries', function ($expectedAmount, Period $period) {
     expect($period->length())->toEqual($expectedAmount);
 })->with('periods_with_amounts_of_included_dates');
 
+it('length is zero when excluding boundaries leaves nothing', function (Period $period) {
+    expect(iterator_count($period))->toEqual(0);
+    expect($period->length())->toEqual(0);
+})->with('periods_left_empty_by_excluded_boundaries');
+
 it('overlap with excluded boundaries', function () {
     $a = Period::make('2018-01-01', '2018-01-05', boundaries: Boundaries::EXCLUDE_END());
     $b = Period::make('2018-01-05', '2018-01-10');
diff --git a/tests/Datasets/Dates.php b/tests/Datasets/Dates.php
index ef9154a..ee7bd17 100644
--- a/tests/Datasets/Dates.php
+++ b/tests/Datasets/Dates.php
@@ -81,6 +81,38 @@ dataset('periods_with_amounts_of_included_dates', function () {
     yield [86362, Period::make('2018-01-01 00:00:00', '2018-01-01 23:59:22', Precision::SECOND(), Boundaries::EXCLUDE_START())];
     yield [86362, Period::make('2018-01-01 00:00:00', '2018-01-01 23:59:22', Precision::SECOND(), Boundaries::EXCLUDE_END())];
     yield [86361, Period::make('2018-01-01 00:00:00', '2018-01-01 23:59:22', Precision::SECOND(), Boundaries::EXCLUDE_ALL())];
+
+    // Excluding a boundary of a one unit long period leaves nothing behind.
+    yield [1, Period::make('2021-01-01', '2021-01-01', Precision::DAY(), Boundaries::EXCLUDE_NONE())];
+    yield [0, Period::make('2021-01-01', '2021-01-01', Precision::DAY(), Boundaries::EXCLUDE_START())];
+    yield [0, Period::make('2021-01-01', '2021-01-01', Precision::DAY(), Boundaries::EXCLUDE_END())];
+    yield [0, Period::make('2021-01-01', '2021-01-01', Precision::DAY(), Boundaries::EXCLUDE_ALL())];
+
+    yield [1, Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::HOUR(), Boundaries::EXCLUDE_NONE())];
+    yield [0, Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::HOUR(), Boundaries::EXCLUDE_START())];
+    yield [0, Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::HOUR(), Boundaries::EXCLUDE_END())];
+    yield [0, Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::HOUR(), Boundaries::EXCLUDE_ALL())];
+
+    yield [0, Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::MINUTE(), Boundaries::EXCLUDE_ALL())];
+    yield [0, Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::SECOND(), Boundaries::EXCLUDE_ALL())];
+
+    // A two unit long period keeps exactly one unit when one of its boundaries is excluded.
+    yield [2, Period::make('2021-01-01', '2021-01-02', Precision::DAY(), Boundaries::EXCLUDE_NONE())];
+    yield [1, Period::make('2021-01-01', '2021-01-02', Precision::DAY(), Boundaries::EXCLUDE_START())];
+    yield [1, Period::make('2021-01-01', '2021-01-02', Precision::DAY(), Boundaries::EXCLUDE_END())];
+    yield [0, Period::make('2021-01-01', '2021-01-02', Precision::DAY(), Boundaries::EXCLUDE_ALL())];
+});
+
+dataset('periods_left_empty_by_excluded_boundaries', function () {
+    yield 'day' => [Period::make('2021-01-01', '2021-01-01', Precision::DAY(), Boundaries::EXCLUDE_START())];
+    yield 'day, end excluded' => [Period::make('2021-01-01', '2021-01-01', Precision::DAY(), Boundaries::EXCLUDE_END())];
+    yield 'day, all excluded' => [Period::make('2021-01-01', '2021-01-01', Precision::DAY(), Boundaries::EXCLUDE_ALL())];
+    yield 'two days, all excluded' => [Period::make('2021-01-01', '2021-01-02', Precision::DAY(), Boundaries::EXCLUDE_ALL())];
+    yield 'hour' => [Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::HOUR(), Boundaries::EXCLUDE_ALL())];
+    yield 'minute' => [Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::MINUTE(), Boundaries::EXCLUDE_ALL())];
+    yield 'second' => [Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::SECOND(), Boundaries::EXCLUDE_ALL())];
+    yield 'month' => [Period::make('2021-01-01', '2021-01-01', Precision::MONTH(), Boundaries::EXCLUDE_ALL())];
+    yield 'year' => [Period::make('2021-01-01', '2021-01-01', Precision::YEAR(), Boundaries::EXCLUDE_ALL())];
 });
 
 dataset('ceiling_dates', function () {