Fix --incremental-by-files skipping updated items east of UTC

github_backup/github_backup.py

@@ -20,7 +20,7 @@
 import time
 import traceback
 from collections.abc import Generator
-from datetime import datetime
+from datetime import datetime, timezone
 from http.client import IncompleteRead
 from urllib.error import HTTPError, URLError
 from urllib.parse import urlencode, urlparse
@@ -2004,7 +2004,7 @@ def get_repository_checkpoint_time(repository):
     if timestamps:
         return max(timestamps)
 
-    return time.strftime("%Y-%m-%dT%H:%M:%SZ", time.localtime())
+    return time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime())
 
 
 def resource_backup_exists(resource_cwd):
@@ -2525,7 +2525,9 @@ def backup_discussions(args, repo_cwd, repository):
 
         if args.incremental_by_files and os.path.isfile(discussion_file):
             modified = os.path.getmtime(discussion_file)
-            modified = datetime.fromtimestamp(modified).strftime("%Y-%m-%dT%H:%M:%SZ")
+            modified = datetime.fromtimestamp(modified, timezone.utc).strftime(
+                "%Y-%m-%dT%H:%M:%SZ"
+            )
             if modified > summary["updatedAt"]:
                 logger.info(
                     "Skipping discussion {0} because it wasn't modified since last backup".format(
@@ -2805,7 +2807,9 @@ def backup_issues(args, repo_cwd, repository, repos_template):
             and number not in stale_timelines
         ):
             modified = os.path.getmtime(issue_file)
-            modified = datetime.fromtimestamp(modified).strftime("%Y-%m-%dT%H:%M:%SZ")
+            modified = datetime.fromtimestamp(modified, timezone.utc).strftime(
+                "%Y-%m-%dT%H:%M:%SZ"
+            )
             if modified > issue["updated_at"]:
                 logger.info(
                     "Skipping issue {0} because it wasn't modified since last backup".format(
@@ -3006,7 +3010,9 @@ def pull_is_due_for_repository_checkpoint(pull):
 
         if args.incremental_by_files and os.path.isfile(pull_file):
             modified = os.path.getmtime(pull_file)
-            modified = datetime.fromtimestamp(modified).strftime("%Y-%m-%dT%H:%M:%SZ")
+            modified = datetime.fromtimestamp(modified, timezone.utc).strftime(
+                "%Y-%m-%dT%H:%M:%SZ"
+            )
             if modified > pull["updated_at"] and not needs_review_backfill:
                 logger.info(
                     "Skipping pull request {0} because it wasn't modified since last backup".format(

tests/test_incremental_by_files.py

@@ -0,0 +1,130 @@
+"""Tests for --incremental-by-files timestamp comparison.
+
+A saved file is skipped when its modification time is newer than the item's
+``updated_at``. GitHub's timestamps are UTC, so the file's modification time
+must be compared in UTC too, whatever the local time zone is.
+"""
+
+import calendar
+import json
+import os
+import time
+
+import pytest
+
+from github_backup import github_backup
+
+UPDATED_AT = "2026-07-01T12:00:00Z"
+
+
+@pytest.fixture
+def east_of_utc(monkeypatch):
+    """Run the test with the local time zone at UTC+10 (no daylight saving)."""
+    monkeypatch.setenv("TZ", "Etc/GMT-10")
+    time.tzset()
+    yield
+    monkeypatch.undo()
+    time.tzset()
+
+
+def _stored(path, number, written_at):
+    """Save an item file whose modification time is ``written_at`` (UTC)."""
+    path.mkdir(parents=True, exist_ok=True)
+    item_file = path / "{0}.json".format(number)
+    item_file.write_text(json.dumps({"number": number, "title": "stored"}))
+    mtime = calendar.timegm(time.strptime(written_at, "%Y-%m-%dT%H:%M:%SZ"))
+    os.utime(item_file, (mtime, mtime))
+    return item_file
+
+
+def _fake_retrieve(items):
+    def _retrieve(args, template, query_args=None, **kwargs):
+        return items if template.endswith(("/issues", "/pulls")) else []
+
+    return _retrieve
+
+
+@pytest.mark.parametrize(
+    "written_at, refetched",
+    [
+        ("2026-07-01T10:00:00Z", True),  # updated 2 hours after it was saved
+        ("2026-07-01T14:00:00Z", False),  # saved 2 hours after the update
+    ],
+)
+def test_issue_skip_compares_in_utc(
+    create_args, tmp_path, monkeypatch, east_of_utc, written_at, refetched
+):
+    issue_file = _stored(tmp_path / "issues", 1, written_at)
+    issue = {"number": 1, "title": "updated", "updated_at": UPDATED_AT}
+    monkeypatch.setattr(github_backup, "retrieve_data", _fake_retrieve([issue]))
+    args = create_args(include_issues=True, incremental_by_files=True, since=None)
+
+    github_backup.backup_issues(
+        args, str(tmp_path), {"full_name": "owner/repo"}, "https://api.github.com/repos"
+    )
+
+    title = json.loads(issue_file.read_text())["title"]
+    assert title == ("updated" if refetched else "stored")
+
+
+@pytest.mark.parametrize(
+    "written_at, refetched",
+    [
+        ("2026-07-01T10:00:00Z", True),
+        ("2026-07-01T14:00:00Z", False),
+    ],
+)
+def test_pull_skip_compares_in_utc(
+    create_args, tmp_path, monkeypatch, east_of_utc, written_at, refetched
+):
+    pull_file = _stored(tmp_path / "pulls", 1, written_at)
+    pull = {"number": 1, "title": "updated", "updated_at": UPDATED_AT}
+    monkeypatch.setattr(github_backup, "retrieve_data", _fake_retrieve([pull]))
+    args = create_args(include_pulls=True, incremental_by_files=True, since=None)
+
+    github_backup.backup_pulls(
+        args, str(tmp_path), {"full_name": "owner/repo"}, "https://api.github.com/repos"
+    )
+
+    title = json.loads(pull_file.read_text())["title"]
+    assert title == ("updated" if refetched else "stored")
+
+
+@pytest.mark.parametrize(
+    "written_at, refetched",
+    [
+        ("2026-07-01T10:00:00Z", True),
+        ("2026-07-01T14:00:00Z", False),
+    ],
+)
+def test_discussion_skip_compares_in_utc(
+    create_args, tmp_path, monkeypatch, east_of_utc, written_at, refetched
+):
+    discussion_file = _stored(tmp_path / "discussions", 1, written_at)
+    summary = {"number": 1, "updatedAt": UPDATED_AT}
+    monkeypatch.setattr(
+        github_backup,
+        "retrieve_discussion_summaries",
+        lambda args, repository, since=None: ([summary], UPDATED_AT, True, 1),
+    )
+    monkeypatch.setattr(
+        github_backup,
+        "retrieve_discussion",
+        lambda args, repository, number: {"number": number, "title": "updated"},
+    )
+    args = create_args(
+        include_discussions=True, incremental_by_files=True, token_classic="ghp_test"
+    )
+
+    github_backup.backup_discussions(args, str(tmp_path), {"full_name": "owner/repo"})
+
+    title = json.loads(discussion_file.read_text())["title"]
+    assert title == ("updated" if refetched else "stored")
+
+
+def test_checkpoint_fallback_time_is_utc(east_of_utc):
+    before = time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime())
+    checkpoint = github_backup.get_repository_checkpoint_time({})
+    after = time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime())
+
+    assert before <= checkpoint <= after