diff --git a/src/borg/testsuite/helpers.py b/src/borg/testsuite/helpers.py
index eab639c36..74f0aeb17 100644
--- a/src/borg/testsuite/helpers.py
+++ b/src/borg/testsuite/helpers.py
@@ -2,6 +2,7 @@ import hashlib
 import os
 import shutil
 import sys
+import time
 from argparse import ArgumentTypeError
 from datetime import datetime, timezone, timedelta
 from io import StringIO, BytesIO
@@ -392,6 +393,12 @@ class MockArchive:
     def __repr__(self):
         return f"{self.id}: {self.ts.isoformat()}"
 
+@pytest.fixture
+def set_prune_test_timezone(monkeypatch):
+    """Run prune tests in a specific full hour timezone"""
+    monkeypatch.setenv("TZ", "Etc/GMT-1")
+    if hasattr(time, "tzset"):  # Only call tzset() on Unix
+        time.tzset()
 
 @pytest.mark.parametrize(
     "rule,num_to_keep,expected_ids", [
@@ -399,13 +406,15 @@ class MockArchive:
         ("monthly", 3, (13, 8, 4)),
         ("weekly", 2, (13, 8)),
         ("daily", 3, (13, 8, 7)),
+        # the "hourly" test case fails if the timezone is not a full-hour offset from UTC. 
+        # therefore, timezone is set to Etc/GMT-1 in set_prune_test_timezone.
         ("hourly", 3, (13, 10, 8)),
         ("minutely", 3, (13, 10, 9)),
         ("secondly", 4, (13, 12, 11, 10)),
         ("daily", 0, []),
     ]
 )
-def test_prune_split(rule, num_to_keep, expected_ids):
+def test_prune_split(rule, num_to_keep, expected_ids, set_prune_test_timezone):
     def subset(lst, ids):
         return {i for i in lst if i.id in ids}