docs/architecture/anniversary-matching.md

@@ -28,8 +28,13 @@ IANA timezone, defaulting to `Europe/Berlin` in the matcher.
 # Leap-Day Behavior
 
 A February 29 post matches February 29 when the current year is a leap year.
-In a non-leap year it matches March 1, preserving one anniversary event for
-leap-day posts.
+In a non-leap year it matches March 1, but only when the post's year is
+exactly ten years before the current year (`today.year − 10`). Because
+`today.year − 10` is never itself a leap year whenever today is a leap year
+(`10 mod 4 == 2`), the Feb 29 ↔ Feb 29 branch is reached independently of
+the year-equality check: any Feb 29 post from a prior leap year is a
+candidate, and its canonical URL keeps the original `YYYY/02/29/<slug>/`
+path.
 
 # Output and Integration
 

src/tenbackward/matching.py

@@ -90,12 +90,16 @@ def _is_unpublished(metadata: dict) -> bool:
 
 
 def _matches_anniversary(post_date: date, target_year: int, today: date) -> bool:
-    if post_date.year != target_year:
-        return False
     if post_date.month == 2 and post_date.day == 29:
         if _is_leap_year(today.year):
             return today.month == 2 and today.day == 29
-        return today.month == 3 and today.day == 1
+        return (
+            post_date.year == target_year
+            and today.month == 3
+            and today.day == 1
+        )
+    if post_date.year != target_year:
+        return False
     return post_date.month == today.month and post_date.day == today.day
 
 

tests/test_matching.py

@@ -133,6 +133,13 @@ def test_iter_anniversary_paths_yields_only_path(tmp_path: Path) -> None:
         # (2016) does not equal target_year (2015), so it does NOT match under
         # the strict year-equality rule.
         (date(2025, 3, 1), "2016-02-29", False),
+        # Feb 29 source (2016) in a leap current year (2028), today is Feb 29:
+        # target_year (2018) is not a leap year, but the Feb 29 -> Feb 29
+        # leap-year branch accepts any prior Feb 29 post.
+        (date(2028, 2, 29), "2016-02-29", True),
+        # Feb 29 source (2020) in a leap current year (2028), today is Feb 29:
+        # same branch, also matches.
+        (date(2028, 2, 29), "2020-02-29", True),
     ],
 )
 def test_leap_day_matching(
@@ -140,7 +147,8 @@ def test_leap_day_matching(
 ) -> None:
     root = tmp_path / "_posts" / "blog"
     target_year = today.year - 10
-    rel = f"{target_year}/{source_iso}-leap.md"
+    post_year = int(source_iso.split("-", 1)[0])
+    rel = f"{post_year}/{source_iso}-leap.md"
     _write_post(
         root,
         rel,
@@ -155,6 +163,32 @@ def test_leap_day_matching(
         assert rel not in matched_paths
 
 
+def test_leap_day_feb29_matches_with_canonical_url(tmp_path: Path) -> None:
+    root = tmp_path / "_posts" / "blog"
+    rel = "2016/2016-02-29-ten-years-before-2026.md"
+    _write_post(
+        root,
+        rel,
+        body=(
+            "---\n"
+            "title: Would Be Ten Years Ago Today In 2026 Mar 1\n"
+            "date: 2016-02-29 08:00:00\n"
+            "---\n"
+            "body\n"
+        ),
+    )
+
+    today = date(2028, 2, 29)
+    matches = find_anniversary_matches(root, SITE_URL, today=today)
+
+    assert len(matches) == 1
+    match = matches[0]
+    assert match.path == rel
+    assert match.date == date(2016, 2, 29)
+    assert match.title == "Would Be Ten Years Ago Today In 2026 Mar 1"
+    assert match.url == "https://chaospott.de/2016/02/29/would-be-ten-years-ago-today-in-2026-mar-1/"
+
+
 def test_match_uses_datetime_frontmatter_date(tmp_path: Path) -> None:
     root = tmp_path / "_posts" / "blog"
     _write_post(