diff --git a/github_activity/github_activity.py b/github_activity/github_activity.py
index 45d483b..33a9413 100644
--- a/github_activity/github_activity.py
+++ b/github_activity/github_activity.py
@@ -605,8 +605,13 @@ def filter_ignored(userlist):
     until_dt_str = data.until_dt_str  # noqa: F841
 
     # Separate into closed and opened
-    closed = data.query("closedAt >= @since_dt_str and closedAt <= @until_dt_str")
-    opened = data.query("createdAt >= @since_dt_str and createdAt <= @until_dt_str")
+    # Git-ref --since is exclusive of the tagged merge; a date --since is inclusive.
+    if data.since_is_git_ref:
+        closed = data.query("closedAt > @since_dt_str and closedAt <= @until_dt_str")
+        opened = data.query("createdAt > @since_dt_str and createdAt <= @until_dt_str")
+    else:
+        closed = data.query("closedAt >= @since_dt_str and closedAt <= @until_dt_str")
+        opened = data.query("createdAt >= @since_dt_str and createdAt <= @until_dt_str")
 
     # Separate into PRs and issues
     closed_prs = closed.query("kind == 'pr'")
diff --git a/tests/test_activity_window.py b/tests/test_activity_window.py
new file mode 100644
index 0000000..a30c897
--- /dev/null
+++ b/tests/test_activity_window.py
@@ -0,0 +1,119 @@
+"""Tests for changelog activity window filtering."""
+
+import datetime
+
+import pandas as pd
+
+from github_activity.github_activity import generate_activity_md
+
+
+def _pr_row(number, title, closed_at, labels=None, created_at=None, merge_oid=None):
+    return {
+        "id": f"PR_{number}",
+        "title": title,
+        "url": f"https://github.com/jupyterhub/action-k3s-helm/pull/{number}",
+        "number": number,
+        "state": "MERGED",
+        "kind": "pr",
+        "org": "jupyterhub",
+        "repo": "action-k3s-helm",
+        "author": "manics",
+        "mergedBy": "manics",
+        "committers": ["manics"],
+        "reviewers": [],
+        "comments": {"edges": []},
+        "labels": labels or [],
+        "closedAt": closed_at,
+        "createdAt": created_at or "2022-08-20T10:00:00Z",
+        "mergeCommit": {"oid": merge_oid or f"sha{number}"},
+        "baseRefName": "master",
+    }
+
+
+def _activity_frame(rows, since_dt, until_dt, since_is_git_ref, until_is_git_ref=False):
+    data = pd.DataFrame(rows)
+    data.since_dt = since_dt
+    data.until_dt = until_dt
+    data.since_dt_str = f"{since_dt:%Y-%m-%dT%H:%M:%SZ}"
+    data.until_dt_str = f"{until_dt:%Y-%m-%dT%H:%M:%SZ}"
+    data.since_is_git_ref = since_is_git_ref
+    data.until_is_git_ref = until_is_git_ref
+    data.attrs["bot_users"] = set()
+    return data
+
+
+def test_git_ref_since_excludes_pr_closed_at_tag(monkeypatch):
+    """A PR whose merge commit is the --since tag must not appear in the changelog.
+
+    GitHub compare `tag...HEAD` is exclusive of the tag. Matching that window
+    means a PR closed at the tagged commit's timestamp belongs to the previous
+    release. Regression for executablebooks/github-activity#79.
+    """
+    since_dt = datetime.datetime(2022, 8, 21, 13, 29, 31, tzinfo=datetime.timezone.utc)
+    until_dt = datetime.datetime(2022, 11, 15, 19, 11, 20, tzinfo=datetime.timezone.utc)
+    data = _activity_frame(
+        [
+            _pr_row(
+                72,
+                "Earlier same-day PR",
+                "2022-08-21T10:00:00Z",
+                labels=["bug"],
+            ),
+            _pr_row(
+                73,
+                "Calico 3.24.0 (3.23 manifest URL is broken)",
+                "2022-08-21T13:29:31Z",
+                labels=["bug"],
+                merge_oid="d3c165a02b1c75b6e551da97970eecb401269750",
+            ),
+            _pr_row(
+                74,
+                "Add RELEASE.md",
+                "2022-08-21T15:22:38Z",
+            ),
+        ],
+        since_dt,
+        until_dt,
+        since_is_git_ref=True,
+        until_is_git_ref=True,
+    )
+    monkeypatch.setattr(
+        "github_activity.github_activity.get_activity", lambda *args, **kwargs: data
+    )
+
+    md = generate_activity_md(
+        target="jupyterhub/action-k3s-helm",
+        since="v3.0.5",
+        until="v3.0.6",
+    )
+
+    assert "[#73]" not in md
+    assert "[#72]" not in md
+    assert "[#74]" in md
+
+
+def test_date_since_still_includes_prs_at_window_start(monkeypatch):
+    """A calendar-date --since remains inclusive of activity at that instant."""
+    since_dt = datetime.datetime(2022, 8, 21, 0, 0, 0, tzinfo=datetime.timezone.utc)
+    until_dt = datetime.datetime(2022, 8, 22, 0, 0, 0, tzinfo=datetime.timezone.utc)
+    data = _activity_frame(
+        [
+            _pr_row(73, "Closed at start of since date", "2022-08-21T00:00:00Z"),
+            _pr_row(74, "Closed later the same day", "2022-08-21T15:22:38Z"),
+        ],
+        since_dt,
+        until_dt,
+        since_is_git_ref=False,
+    )
+    monkeypatch.setattr(
+        "github_activity.github_activity.get_activity", lambda *args, **kwargs: data
+    )
+
+    md = generate_activity_md(
+        target="jupyterhub/action-k3s-helm",
+        since="2022-08-21",
+        until="2022-08-22",
+    )
+
+    assert "[#73]" in md
+    assert "[#74]" in md