diff --git a/bincio/segments/detect.py b/bincio/segments/detect.py
index f7cbb96..af1626c 100644
--- a/bincio/segments/detect.py
+++ b/bincio/segments/detect.py
@@ -197,7 +197,7 @@ def _extract_effort(
     j: int,
 ) -> SegmentEffort:
     elapsed_s = track.times[j] - track.times[i]
-    started_at = track.started_at + timedelta(seconds=track.times[i])
+    started_at = (track.started_at + timedelta(seconds=track.times[i])).replace(microsecond=0)
     avg_speed = _avg_nonnull(track.speeds, i, j)
     avg_hr_raw = _avg_nonnull(track.hrs, i, j)
     avg_hr = int(round(avg_hr_raw)) if avg_hr_raw is not None else None
diff --git a/bincio/segments/store.py b/bincio/segments/store.py
index e9d89fe..833ec66 100644
--- a/bincio/segments/store.py
+++ b/bincio/segments/store.py
@@ -172,10 +172,10 @@ def save_efforts(data_dir: Path, handle: str, segment_id: str, efforts: list[Seg
 
 
 def add_effort(data_dir: Path, handle: str, segment_id: str, effort: SegmentEffort) -> None:
-    """Append an effort, replacing any existing effort for the same activity."""
+    """Append an effort, replacing any existing effort with the same activity + start time."""
     efforts = load_efforts(data_dir, handle, segment_id)
-    efforts = [e for e in efforts if e.activity_id != effort.activity_id or
-               e.started_at != effort.started_at]
+    key = (effort.activity_id, _iso(effort.started_at))
+    efforts = [e for e in efforts if (e.activity_id, _iso(e.started_at)) != key]
     efforts.append(effort)
     efforts.sort(key=lambda e: e.started_at, reverse=True)
     save_efforts(data_dir, handle, segment_id, efforts)
diff --git a/bincio/serve/server.py b/bincio/serve/server.py
index 6af0551..267e309 100644
--- a/bincio/serve/server.py
+++ b/bincio/serve/server.py
@@ -23,7 +23,7 @@ from typing import Any, Optional
 
 log = logging.getLogger("bincio.serve")
 
-from fastapi import Cookie, Depends, FastAPI, File, Form, HTTPException, Request, Response, UploadFile
+from fastapi import BackgroundTasks, Cookie, Depends, FastAPI, File, Form, HTTPException, Request, Response, UploadFile
 from fastapi.responses import FileResponse, RedirectResponse, StreamingResponse
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.middleware.gzip import GZipMiddleware
@@ -2459,6 +2459,7 @@ async def get_segment(segment_id: str) -> JSONResponse:
 @app.post("/api/segments")
 async def create_segment(
     body: CreateSegmentRequest,
+    background_tasks: BackgroundTasks,
     bincio_session: Optional[str] = Cookie(default=None),
 ) -> JSONResponse:
     user = _require_user(bincio_session)
@@ -2483,7 +2484,9 @@ async def create_segment(
         created_by=user.handle,
         created_at=datetime.now(_tz.utc),
     )
-    _seg_store.save_segment(_get_data_dir(), seg)
+    dd = _get_data_dir()
+    _seg_store.save_segment(dd, seg)
+    background_tasks.add_task(_scan_segment_for_user, dd, user.handle, seg_id)
     return JSONResponse({"id": seg_id}, status_code=201)
 
 
@@ -2529,30 +2532,22 @@ async def get_segment_efforts(
     ])
 
 
-@app.post("/api/segments/{segment_id}/detect")
-async def trigger_detect(
-    segment_id: str,
-    bincio_session: Optional[str] = Cookie(default=None),
-) -> JSONResponse:
-    """Retroactively detect efforts on a segment for the logged-in user."""
-    user = _require_user(bincio_session)
-    dd = _get_data_dir()
-    seg = _seg_store.load_segment(dd, segment_id)
-    if seg is None:
-        raise HTTPException(404, "Segment not found")
-
+def _scan_segment_for_user(dd: Path, handle: str, segment_id: str) -> int:
+    """Scan all of a user's activities against one segment. Returns effort count."""
     from datetime import datetime as _datetime
     from bincio.segments.detect import track_from_timeseries_json, detect_one
-    import json as _json
 
-    user_dir = dd / user.handle
+    seg = _seg_store.load_segment(dd, segment_id)
+    if seg is None:
+        return 0
+    user_dir = dd / handle
     acts_dir = user_dir / "activities"
     total = 0
     for detail_path in sorted(acts_dir.glob("*.json")):
         if ".timeseries." in detail_path.name:
             continue
         try:
-            detail = _json.loads(detail_path.read_text(encoding="utf-8"))
+            detail = json.loads(detail_path.read_text(encoding="utf-8"))
         except Exception:
             continue
         ts_url = detail.get("timeseries_url")
@@ -2562,7 +2557,7 @@ async def trigger_detect(
         if not ts_path.exists():
             continue
         try:
-            ts = _json.loads(ts_path.read_text(encoding="utf-8"))
+            ts = json.loads(ts_path.read_text(encoding="utf-8"))
         except Exception:
             continue
         started_raw = detail.get("started_at")
@@ -2578,9 +2573,22 @@ async def trigger_detect(
             continue
         efforts = detect_one(track, seg)
         for effort in efforts:
-            _seg_store.add_effort(dd, user.handle, segment_id, effort)
+            _seg_store.add_effort(dd, handle, segment_id, effort)
         total += len(efforts)
+    return total
 
+
+@app.post("/api/segments/{segment_id}/detect")
+async def trigger_detect(
+    segment_id: str,
+    bincio_session: Optional[str] = Cookie(default=None),
+) -> JSONResponse:
+    """Retroactively detect efforts on a segment for the logged-in user."""
+    user = _require_user(bincio_session)
+    dd = _get_data_dir()
+    if _seg_store.load_segment(dd, segment_id) is None:
+        raise HTTPException(404, "Segment not found")
+    total = _scan_segment_for_user(dd, user.handle, segment_id)
     return JSONResponse({"ok": True, "efforts_found": total})