diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..49a4bf8
--- /dev/null
+++ b/tests/__init__.py
@@ -0,0 +1,3 @@
+from . import test_format_duration
+
+__all__ = ["test_format_duration"]
diff --git a/tests/test_format_duration.py b/tests/test_format_duration.py
new file mode 100644
index 0000000..4b8752a
--- /dev/null
+++ b/tests/test_format_duration.py
@@ -0,0 +1,50 @@
+import unittest
+
+import utils
+
+
+class TestFormatDuration(unittest.TestCase):
+    def test_default(self):
+        self.assertEqual(utils.format_duration(0), "")
+        self.assertEqual(utils.format_duration(60 * 60 * 24 * 7), "1 week")
+        self.assertEqual(utils.format_duration(60 * 60 * 24 * 21), "3 weeks")
+        self.assertEqual(
+            utils.format_duration((60 * 60 * 24 * 21) - 1),
+            "2 weeks, 6 days, 23 hours, 59 minutes, 59 seconds",
+        )
+        self.assertEqual(utils.format_duration(60), "1 minute")
+        self.assertEqual(utils.format_duration(60 * 2), "2 minutes")
+        self.assertEqual(utils.format_duration(60 * 59), "59 minutes")
+        self.assertEqual(utils.format_duration(60 * 60), "1 hour")
+        self.assertEqual(utils.format_duration(60 * 60 * 2), "2 hours")
+        self.assertEqual(utils.format_duration(1), "1 second")
+        self.assertEqual(utils.format_duration(60 + 5), "1 minute, 5 seconds")
+        self.assertEqual(utils.format_duration(60 * 60 + 30), "1 hour, 30 seconds")
+        self.assertEqual(
+            utils.format_duration(60 * 60 + 60 + 30), "1 hour, 1 minute, 30 seconds"
+        )
+        self.assertEqual(
+            utils.format_duration(60 * 60 * 24 * 7 + 30), "1 week, 30 seconds"
+        )
+
+    def test_natural(self):
+        def format(seconds: int):
+            return utils.format_duration(seconds, natural=True)
+
+        self.assertEqual(format(0), "")
+        self.assertEqual(format(60 * 60 * 24 * 7), "1 week")
+        self.assertEqual(format(60 * 60 * 24 * 21), "3 weeks")
+        self.assertEqual(
+            format((60 * 60 * 24 * 21) - 1),
+            "2 weeks, 6 days, 23 hours, 59 minutes and 59 seconds",
+        )
+        self.assertEqual(format(60), "1 minute")
+        self.assertEqual(format(60 * 2), "2 minutes")
+        self.assertEqual(format(60 * 59), "59 minutes")
+        self.assertEqual(format(60 * 60), "1 hour")
+        self.assertEqual(format(60 * 60 * 2), "2 hours")
+        self.assertEqual(format(1), "1 second")
+        self.assertEqual(format(60 + 5), "1 minute and 5 seconds")
+        self.assertEqual(format(60 * 60 + 30), "1 hour and 30 seconds")
+        self.assertEqual(format(60 * 60 + 60 + 30), "1 hour, 1 minute and 30 seconds")
+        self.assertEqual(format(60 * 60 * 24 * 7 + 30), "1 week and 30 seconds")