diff --git a/src/xai_sdk/poll_timer.py b/src/xai_sdk/poll_timer.py
index 9e71c76..8f0b5d7 100644
--- a/src/xai_sdk/poll_timer.py
+++ b/src/xai_sdk/poll_timer.py
@@ -24,7 +24,7 @@ def __init__(
             context: A description of what is being waited on (e.g. "waiting for document to be indexed").
                 Included in the TimeoutError message for easier debugging.
         """
-        self._start = time.time()
+        self._start = time.monotonic()
         self._timeout = timeout or datetime.timedelta(minutes=10)
         self._interval = interval or datetime.timedelta(seconds=1)
         self._context = context
@@ -38,7 +38,7 @@ def sleep_interval_or_raise(self) -> float:
         Raises:
             TimeoutError when the total polling time is used up.
         """
-        runtime = time.time() - self._start
+        runtime = time.monotonic() - self._start
         if runtime > self._timeout.total_seconds():
             message = f"Polling timed out after {runtime:.1f}s"
             if self._context:
diff --git a/tests/poll_timer_test.py b/tests/poll_timer_test.py
new file mode 100644
index 0000000..4695b2e
--- /dev/null
+++ b/tests/poll_timer_test.py
@@ -0,0 +1,51 @@
+import datetime
+from unittest.mock import patch
+
+import pytest
+
+from xai_sdk.poll_timer import PollTimer
+
+
+def test_sleep_interval_or_raise_unaffected_by_wall_clock_jump_forward():
+    """Wall-clock jumps must not affect timeout accounting (uses monotonic)."""
+    with (
+        patch("xai_sdk.poll_timer.time.time", side_effect=[100.0, 10_000.0]),
+        patch("xai_sdk.poll_timer.time.monotonic", side_effect=[0.0, 1.0]),
+    ):
+        timer = PollTimer(
+            timeout=datetime.timedelta(seconds=10),
+            interval=datetime.timedelta(seconds=20),
+        )
+        assert timer.sleep_interval_or_raise() == pytest.approx(9.0)
+
+
+def test_sleep_interval_or_raise_unaffected_by_wall_clock_jump_backward():
+    with (
+        patch("xai_sdk.poll_timer.time.time", side_effect=[10_000.0, 100.0]),
+        patch("xai_sdk.poll_timer.time.monotonic", side_effect=[0.0, 2.0]),
+    ):
+        timer = PollTimer(
+            timeout=datetime.timedelta(seconds=10),
+            interval=datetime.timedelta(seconds=20),
+        )
+        assert timer.sleep_interval_or_raise() == pytest.approx(8.0)
+
+
+def test_sleep_interval_or_raise_times_out_based_on_monotonic_elapsed():
+    with patch("xai_sdk.poll_timer.time.monotonic", side_effect=[0.0, 11.0]):
+        timer = PollTimer(
+            timeout=datetime.timedelta(seconds=10),
+            interval=datetime.timedelta(seconds=1),
+            context="waiting for document to be indexed",
+        )
+        with pytest.raises(TimeoutError, match=r"Polling timed out after 11\.0s: waiting for document"):
+            timer.sleep_interval_or_raise()
+
+
+def test_sleep_interval_or_raise_returns_min_of_remaining_and_interval():
+    with patch("xai_sdk.poll_timer.time.monotonic", side_effect=[0.0, 0.5]):
+        timer = PollTimer(
+            timeout=datetime.timedelta(seconds=10),
+            interval=datetime.timedelta(seconds=1),
+        )
+        assert timer.sleep_interval_or_raise() == pytest.approx(1.0)