From 36dce79934614ae4d19c019152b2738d7c92d07a Mon Sep 17 00:00:00 2001
From: Christopher Krah <48338417+chrikrah@users.noreply.github.com>
Date: Sun, 8 Nov 2026 11:09:02 +0100
Subject: [PATCH 1/2] Test parsing a negative timestamp between -1 and 0

Signed-off-by: Christopher Krah <48338417+chrikrah@users.noreply.github.com>
---
 tests/openmetrics/test_parser.py | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/tests/openmetrics/test_parser.py b/tests/openmetrics/test_parser.py
index 684d9a82..78b7af83 100644
--- a/tests/openmetrics/test_parser.py
+++ b/tests/openmetrics/test_parser.py
@@ -627,6 +627,20 @@ def test_null_byte(self):
         metric_family = CounterMetricFamily("a", "he\0lp")
         self.assertEqual([metric_family], list(families))
 
+    def test_negative_subsecond_timestamps(self):
+        families = text_string_to_metric_families("""# TYPE a counter
+# HELP a help
+a_total{foo="1"} 1 -0.500000000
+a_total{foo="2"} 1 -1.500000000 # {a="b"} 0.5 -0.250000000
+# EOF
+""")
+        a = CounterMetricFamily("a", "help", labels=["foo"])
+        a.add_metric(["1"], 1, timestamp=Timestamp(-0.5, 500000000))
+        a.add_metric(["2"], 1, timestamp=Timestamp(-1, 500000000))
+        a.samples[1] = a.samples[1]._replace(exemplar=Exemplar({"a": "b"}, 0.5, Timestamp(-0.25, 250000000)))
+        self.assertEqual([a], list(families))
+        self.assertEqual("-0.500000000", str(a.samples[0].timestamp))
+
     def test_timestamps(self):
         families = text_string_to_metric_families("""# TYPE a counter
 # HELP a help

From 3bb5be2d7adcaf401bc667207b3ccadd38ca4819 Mon Sep 17 00:00:00 2001
From: Christopher Krah <48338417+chrikrah@users.noreply.github.com>
Date: Sun, 8 Nov 2026 11:09:02 +0100
Subject: [PATCH 2/2] Keep the sign of a negative timestamp between -1 and 0 in
 the OpenMetrics parser

Signed-off-by: Christopher Krah <48338417+chrikrah@users.noreply.github.com>
---
 prometheus_client/openmetrics/parser.py | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/prometheus_client/openmetrics/parser.py b/prometheus_client/openmetrics/parser.py
index adb3a388..ebd6da10 100644
--- a/prometheus_client/openmetrics/parser.py
+++ b/prometheus_client/openmetrics/parser.py
@@ -91,7 +91,12 @@ def _parse_timestamp(timestamp):
         try:
             # aaaa.bbbb. Nanosecond resolution supported.
             parts = timestamp.split('.', 1)
-            return Timestamp(int(parts[0]), int(parts[1][:9].ljust(9, "0")))
+            sec = int(parts[0])
+            nsec = int(parts[1][:9].ljust(9, "0"))
+            if sec == 0 and parts[0].startswith('-'):
+                # int("-0") is 0, so carry the sign the way Timestamp(-0.5, 500000000) does.
+                return Timestamp(-nsec / 1e9, nsec)
+            return Timestamp(sec, nsec)
         except ValueError:
             # Float.
             ts = float(timestamp)