diff --git a/retro/SKILL.md b/retro/SKILL.md
index 92d58f7b8e..2bc9cc7245 100644
--- a/retro/SKILL.md
+++ b/retro/SKILL.md
@@ -823,7 +823,7 @@ When the user types `/retro`, run this skill.
 
 Parse the argument to determine the time window. Default to 7 days if no argument given. All times should be reported in the user's **local timezone** (use the system default — do NOT set `TZ`).
 
-**Midnight-aligned windows:** For day (`d`) and week (`w`) units, compute an absolute start date at local midnight, not a relative string. For example, if today is 2026-03-18 and the window is 7 days: the start date is 2026-03-11. Use `--since="2026-03-11T00:00:00"` for git log queries — the explicit `T00:00:00` suffix ensures git starts from midnight. Without it, git uses the current wall-clock time (e.g., `--since="2026-03-11"` at 11pm means 11pm, not midnight). For week units, multiply by 7 to get days (e.g., `2w` = 14 days back). For hour (`h`) units, use `--since="N hours ago"` since midnight alignment does not apply to sub-day windows.
+**Midnight-aligned windows:** Treat the shell system date (`date +%Y-%m-%d`) as the source of truth for "today"; do not derive today from session reminders, model memory, or prior conversation context. For day (`d`) and week (`w`) units, compute an absolute start date at local midnight, not a relative string. For example, if the system date is 2026-03-18 and the window is 7 days: the start date is 2026-03-11. Use `--since="2026-03-11T00:00:00"` for git log queries — the explicit `T00:00:00` suffix ensures git starts from midnight. Without it, git uses the current wall-clock time (e.g., `--since="2026-03-11"` at 11pm means 11pm, not midnight). For week units, multiply by 7 to get days (e.g., `2w` = 14 days back). For hour (`h`) units, use `--since="N hours ago"` since midnight alignment does not apply to sub-day windows.
 
 **Argument validation:** If the argument doesn't match a number followed by `d`, `h`, or `w`, the word `compare` (optionally followed by a window), or the word `global` (optionally followed by a window), show this usage and stop:
 ```
@@ -888,11 +888,50 @@ Check for non-git context that should be included in the retro:
 
 If `RETRO_CONTEXT_FOUND`: read `~/.gstack/retro-context.md`. This file is user-authored and may contain meeting notes, calendar events, decisions, and other context that doesn't appear in git history. Incorporate this context into the retro narrative where relevant.
 
-### Step 1: Gather Raw Data
+### Date/window pre-flight
+
+Run this dedicated pre-flight after resolving the requested window and before the main data-gathering commands. For `/retro compare`, run it against the current window before computing the prior window. For `/retro global`, run it once per discovered repo after detecting that repo's default branch and before collecting its git-log data.
+
+Resolve:
+- `<window>` to the exact `--since` value used for the current window, such as `2026-05-13T00:00:00`.
+- `<window_days>` to the numeric current-window length in days. Use `7` for the default window, `N` for `Nd`, `N * 7` for `Nw`, and `ceil(N / 24)` with a minimum of `1` for `Nh`.
 
-First, fetch origin and identify the current user:
 ```bash
 git fetch origin <default> --quiet
+
+_RETRO_SYSTEM_DATE=$(date +%Y-%m-%d)
+_RETRO_WINDOW_DAYS="<window_days>"
+case "$_RETRO_WINDOW_DAYS" in ''|*[!0-9]*) _RETRO_WINDOW_DAYS=7 ;; esac
+if [ "$_RETRO_WINDOW_DAYS" -lt 1 ]; then _RETRO_WINDOW_DAYS=1; fi
+_ORIGIN_LATEST_UNIX=$(git log origin/<default> -1 --format="%at" 2>/dev/null || echo 0)
+_ORIGIN_LATEST_DATE=$(git log origin/<default> -1 --format="%ad" --date=short 2>/dev/null || echo unknown)
+case "$_ORIGIN_LATEST_UNIX" in ''|*[!0-9]*) _ORIGIN_LATEST_UNIX=0 ;; esac
+if [ "$_ORIGIN_LATEST_UNIX" -gt 0 ]; then
+  _ORIGIN_GAP_DAYS=$(( ($(date +%s) - _ORIGIN_LATEST_UNIX) / 86400 ))
+else
+  _ORIGIN_GAP_DAYS=9999
+fi
+_WINDOW_COMMITS=$(git rev-list --count origin/<default> --since="<window>" 2>/dev/null || echo 0)
+case "$_WINDOW_COMMITS" in ''|*[!0-9]*) _WINDOW_COMMITS=0 ;; esac
+echo "RETRO_SYSTEM_DATE: $_RETRO_SYSTEM_DATE"
+echo "RETRO_WINDOW_DAYS: $_RETRO_WINDOW_DAYS"
+echo "ORIGIN_DEFAULT_LATEST: $_ORIGIN_LATEST_DATE (${_ORIGIN_GAP_DAYS}d before system date)"
+echo "RETRO_WINDOW_COMMIT_COUNT: $_WINDOW_COMMITS"
+if [ "$_ORIGIN_GAP_DAYS" -gt "$_RETRO_WINDOW_DAYS" ]; then
+  echo "STALE-BASE WARNING: latest origin/<default> commit is ${_ORIGIN_GAP_DAYS} days before the system date, which is older than the ${_RETRO_WINDOW_DAYS}d retro window. Confirm the 'today' anchor and whether origin/<default> is current before writing the retro."
+fi
+if [ "$_WINDOW_COMMITS" -eq 0 ]; then
+  echo "EMPTY-WINDOW WARNING: origin/<default> has zero commits in the requested retro window. Treat this as a possible wrong today/window anchor, not proof that no work happened."
+fi
+```
+
+If the date you used to compute a day/week window does not match `RETRO_SYSTEM_DATE`, print `TODAY-ANCHOR WARNING`, recompute the window from `RETRO_SYSTEM_DATE`, and rerun the pre-flight. If `STALE-BASE WARNING` or `EMPTY-WINDOW WARNING` appears, call it out before any normal retro narrative. Do not produce a clean-looking "nothing happened" retro without explaining the suspect system date/window/base-branch evidence. This window-aware threshold intentionally catches the #1624 9-day-gap/7d reproducer: if the latest default-branch commit is 9 days before the system date and the current retro window is 7 days, the pre-flight must print `STALE-BASE WARNING`.
+
+### Step 1: Gather Raw Data
+
+First, identify the current user:
+
+```bash
 # Identify who is running the retro
 git config user.name
 git config user.email
@@ -1440,6 +1479,8 @@ git -C <path> fetch origin --quiet 2>/dev/null
 
 Detect the default branch for each repo: first try `git symbolic-ref refs/remotes/origin/HEAD`, then check common branch names (`main`, `master`), then fall back to `git rev-parse --abbrev-ref HEAD`. Use the detected branch as `<default>` in the commands below.
 
+Run the **Date/window pre-flight** for each repo before the commands below. For remote repos, use `origin/$DEFAULT` where that pre-flight says `origin/<default>`. For local-only repos, use `HEAD` in place of `origin/<default>`, skip the fetch line, and label warnings with the repo name. In the global summary, list any repos that emitted `STALE-BASE WARNING` or `EMPTY-WINDOW WARNING` before presenting aggregate totals.
+
 ```bash
 # Commits with stats
 git -C <path> log origin/$DEFAULT --since="<start_date>T00:00:00" --format="%H|%aN|%ai|%s" --shortstat
@@ -1687,11 +1728,12 @@ Use the Write tool to save JSON to `~/.gstack/retros/global-${today}-${next}.jso
 
 When the user runs `/retro compare` (or `/retro compare 14d`):
 
-1. Compute metrics for the current window (default 7d) using the midnight-aligned start date (same logic as the main retro — e.g., if today is 2026-03-18 and window is 7d, use `--since="2026-03-11T00:00:00"`)
-2. Compute metrics for the immediately prior same-length window using both `--since` and `--until` with midnight-aligned dates to avoid overlap (e.g., for a 7d window starting 2026-03-11: prior window is `--since="2026-03-04T00:00:00" --until="2026-03-11T00:00:00"`)
-3. Show a side-by-side comparison table with deltas and arrows
-4. Write a brief narrative highlighting the biggest improvements and regressions
-5. Save only the current-window snapshot to `.context/retros/` (same as a normal retro run); do **not** persist the prior-window metrics.
+1. Run the **Date/window pre-flight** for the current window (default 7d). If it emits `TODAY-ANCHOR WARNING`, `STALE-BASE WARNING`, or `EMPTY-WINDOW WARNING`, show that warning before any comparison output.
+2. Compute metrics for the current window using the midnight-aligned start date (same logic as the main retro — e.g., if today is 2026-03-18 and window is 7d, use `--since="2026-03-11T00:00:00"`)
+3. Compute metrics for the immediately prior same-length window using both `--since` and `--until` with midnight-aligned dates to avoid overlap (e.g., for a 7d window starting 2026-03-11: prior window is `--since="2026-03-04T00:00:00" --until="2026-03-11T00:00:00"`)
+4. Show a side-by-side comparison table with deltas and arrows
+5. Write a brief narrative highlighting the biggest improvements and regressions
+6. Save only the current-window snapshot to `.context/retros/` (same as a normal retro run); do **not** persist the prior-window metrics.
 
 ## Tone
 
diff --git a/retro/SKILL.md.tmpl b/retro/SKILL.md.tmpl
index 93aed3d43e..dc05998693 100644
--- a/retro/SKILL.md.tmpl
+++ b/retro/SKILL.md.tmpl
@@ -66,7 +66,7 @@ When the user types `/retro`, run this skill.
 
 Parse the argument to determine the time window. Default to 7 days if no argument given. All times should be reported in the user's **local timezone** (use the system default — do NOT set `TZ`).
 
-**Midnight-aligned windows:** For day (`d`) and week (`w`) units, compute an absolute start date at local midnight, not a relative string. For example, if today is 2026-03-18 and the window is 7 days: the start date is 2026-03-11. Use `--since="2026-03-11T00:00:00"` for git log queries — the explicit `T00:00:00` suffix ensures git starts from midnight. Without it, git uses the current wall-clock time (e.g., `--since="2026-03-11"` at 11pm means 11pm, not midnight). For week units, multiply by 7 to get days (e.g., `2w` = 14 days back). For hour (`h`) units, use `--since="N hours ago"` since midnight alignment does not apply to sub-day windows.
+**Midnight-aligned windows:** Treat the shell system date (`date +%Y-%m-%d`) as the source of truth for "today"; do not derive today from session reminders, model memory, or prior conversation context. For day (`d`) and week (`w`) units, compute an absolute start date at local midnight, not a relative string. For example, if the system date is 2026-03-18 and the window is 7 days: the start date is 2026-03-11. Use `--since="2026-03-11T00:00:00"` for git log queries — the explicit `T00:00:00` suffix ensures git starts from midnight. Without it, git uses the current wall-clock time (e.g., `--since="2026-03-11"` at 11pm means 11pm, not midnight). For week units, multiply by 7 to get days (e.g., `2w` = 14 days back). For hour (`h`) units, use `--since="N hours ago"` since midnight alignment does not apply to sub-day windows.
 
 **Argument validation:** If the argument doesn't match a number followed by `d`, `h`, or `w`, the word `compare` (optionally followed by a window), or the word `global` (optionally followed by a window), show this usage and stop:
 ```
@@ -95,11 +95,50 @@ Check for non-git context that should be included in the retro:
 
 If `RETRO_CONTEXT_FOUND`: read `~/.gstack/retro-context.md`. This file is user-authored and may contain meeting notes, calendar events, decisions, and other context that doesn't appear in git history. Incorporate this context into the retro narrative where relevant.
 
-### Step 1: Gather Raw Data
+### Date/window pre-flight
+
+Run this dedicated pre-flight after resolving the requested window and before the main data-gathering commands. For `/retro compare`, run it against the current window before computing the prior window. For `/retro global`, run it once per discovered repo after detecting that repo's default branch and before collecting its git-log data.
+
+Resolve:
+- `<window>` to the exact `--since` value used for the current window, such as `2026-05-13T00:00:00`.
+- `<window_days>` to the numeric current-window length in days. Use `7` for the default window, `N` for `Nd`, `N * 7` for `Nw`, and `ceil(N / 24)` with a minimum of `1` for `Nh`.
 
-First, fetch origin and identify the current user:
 ```bash
 git fetch origin <default> --quiet
+
+_RETRO_SYSTEM_DATE=$(date +%Y-%m-%d)
+_RETRO_WINDOW_DAYS="<window_days>"
+case "$_RETRO_WINDOW_DAYS" in ''|*[!0-9]*) _RETRO_WINDOW_DAYS=7 ;; esac
+if [ "$_RETRO_WINDOW_DAYS" -lt 1 ]; then _RETRO_WINDOW_DAYS=1; fi
+_ORIGIN_LATEST_UNIX=$(git log origin/<default> -1 --format="%at" 2>/dev/null || echo 0)
+_ORIGIN_LATEST_DATE=$(git log origin/<default> -1 --format="%ad" --date=short 2>/dev/null || echo unknown)
+case "$_ORIGIN_LATEST_UNIX" in ''|*[!0-9]*) _ORIGIN_LATEST_UNIX=0 ;; esac
+if [ "$_ORIGIN_LATEST_UNIX" -gt 0 ]; then
+  _ORIGIN_GAP_DAYS=$(( ($(date +%s) - _ORIGIN_LATEST_UNIX) / 86400 ))
+else
+  _ORIGIN_GAP_DAYS=9999
+fi
+_WINDOW_COMMITS=$(git rev-list --count origin/<default> --since="<window>" 2>/dev/null || echo 0)
+case "$_WINDOW_COMMITS" in ''|*[!0-9]*) _WINDOW_COMMITS=0 ;; esac
+echo "RETRO_SYSTEM_DATE: $_RETRO_SYSTEM_DATE"
+echo "RETRO_WINDOW_DAYS: $_RETRO_WINDOW_DAYS"
+echo "ORIGIN_DEFAULT_LATEST: $_ORIGIN_LATEST_DATE (${_ORIGIN_GAP_DAYS}d before system date)"
+echo "RETRO_WINDOW_COMMIT_COUNT: $_WINDOW_COMMITS"
+if [ "$_ORIGIN_GAP_DAYS" -gt "$_RETRO_WINDOW_DAYS" ]; then
+  echo "STALE-BASE WARNING: latest origin/<default> commit is ${_ORIGIN_GAP_DAYS} days before the system date, which is older than the ${_RETRO_WINDOW_DAYS}d retro window. Confirm the 'today' anchor and whether origin/<default> is current before writing the retro."
+fi
+if [ "$_WINDOW_COMMITS" -eq 0 ]; then
+  echo "EMPTY-WINDOW WARNING: origin/<default> has zero commits in the requested retro window. Treat this as a possible wrong today/window anchor, not proof that no work happened."
+fi
+```
+
+If the date you used to compute a day/week window does not match `RETRO_SYSTEM_DATE`, print `TODAY-ANCHOR WARNING`, recompute the window from `RETRO_SYSTEM_DATE`, and rerun the pre-flight. If `STALE-BASE WARNING` or `EMPTY-WINDOW WARNING` appears, call it out before any normal retro narrative. Do not produce a clean-looking "nothing happened" retro without explaining the suspect system date/window/base-branch evidence. This window-aware threshold intentionally catches the #1624 9-day-gap/7d reproducer: if the latest default-branch commit is 9 days before the system date and the current retro window is 7 days, the pre-flight must print `STALE-BASE WARNING`.
+
+### Step 1: Gather Raw Data
+
+First, identify the current user:
+
+```bash
 # Identify who is running the retro
 git config user.name
 git config user.email
@@ -624,6 +663,8 @@ git -C <path> fetch origin --quiet 2>/dev/null
 
 Detect the default branch for each repo: first try `git symbolic-ref refs/remotes/origin/HEAD`, then check common branch names (`main`, `master`), then fall back to `git rev-parse --abbrev-ref HEAD`. Use the detected branch as `<default>` in the commands below.
 
+Run the **Date/window pre-flight** for each repo before the commands below. For remote repos, use `origin/$DEFAULT` where that pre-flight says `origin/<default>`. For local-only repos, use `HEAD` in place of `origin/<default>`, skip the fetch line, and label warnings with the repo name. In the global summary, list any repos that emitted `STALE-BASE WARNING` or `EMPTY-WINDOW WARNING` before presenting aggregate totals.
+
 ```bash
 # Commits with stats
 git -C <path> log origin/$DEFAULT --since="<start_date>T00:00:00" --format="%H|%aN|%ai|%s" --shortstat
@@ -871,11 +912,12 @@ Use the Write tool to save JSON to `~/.gstack/retros/global-${today}-${next}.jso
 
 When the user runs `/retro compare` (or `/retro compare 14d`):
 
-1. Compute metrics for the current window (default 7d) using the midnight-aligned start date (same logic as the main retro — e.g., if today is 2026-03-18 and window is 7d, use `--since="2026-03-11T00:00:00"`)
-2. Compute metrics for the immediately prior same-length window using both `--since` and `--until` with midnight-aligned dates to avoid overlap (e.g., for a 7d window starting 2026-03-11: prior window is `--since="2026-03-04T00:00:00" --until="2026-03-11T00:00:00"`)
-3. Show a side-by-side comparison table with deltas and arrows
-4. Write a brief narrative highlighting the biggest improvements and regressions
-5. Save only the current-window snapshot to `.context/retros/` (same as a normal retro run); do **not** persist the prior-window metrics.
+1. Run the **Date/window pre-flight** for the current window (default 7d). If it emits `TODAY-ANCHOR WARNING`, `STALE-BASE WARNING`, or `EMPTY-WINDOW WARNING`, show that warning before any comparison output.
+2. Compute metrics for the current window using the midnight-aligned start date (same logic as the main retro — e.g., if today is 2026-03-18 and window is 7d, use `--since="2026-03-11T00:00:00"`)
+3. Compute metrics for the immediately prior same-length window using both `--since` and `--until` with midnight-aligned dates to avoid overlap (e.g., for a 7d window starting 2026-03-11: prior window is `--since="2026-03-04T00:00:00" --until="2026-03-11T00:00:00"`)
+4. Show a side-by-side comparison table with deltas and arrows
+5. Write a brief narrative highlighting the biggest improvements and regressions
+6. Save only the current-window snapshot to `.context/retros/` (same as a normal retro run); do **not** persist the prior-window metrics.
 
 ## Tone
 
diff --git a/test/gen-skill-docs.test.ts b/test/gen-skill-docs.test.ts
index c594ea4bcd..185f2c030b 100644
--- a/test/gen-skill-docs.test.ts
+++ b/test/gen-skill-docs.test.ts
@@ -1085,6 +1085,22 @@ describe('Retro plan completion section', () => {
     expect(retroSkill).toContain('plan_items_total');
     expect(retroSkill).toContain('Plan Completion This Period');
   });
+
+  test('retro SKILL.md warns on stale base or empty windows', () => {
+    expect(retroSkill).toContain('### Date/window pre-flight');
+    expect(retroSkill).toContain('RETRO_SYSTEM_DATE');
+    expect(retroSkill).toContain('RETRO_WINDOW_DAYS');
+    expect(retroSkill).toContain('ORIGIN_DEFAULT_LATEST');
+    expect(retroSkill).toContain('RETRO_WINDOW_COMMIT_COUNT');
+    expect(retroSkill).toContain('TODAY-ANCHOR WARNING');
+    expect(retroSkill).toContain('STALE-BASE WARNING');
+    expect(retroSkill).toContain('EMPTY-WINDOW WARNING');
+    expect(retroSkill).toContain('$_ORIGIN_GAP_DAYS" -gt "$_RETRO_WINDOW_DAYS');
+    expect(retroSkill).toContain('#1624 9-day-gap/7d reproducer');
+    expect(retroSkill).toContain('For `/retro compare`, run it against the current window');
+    expect(retroSkill).toContain('For `/retro global`, run it once per discovered repo');
+    expect(retroSkill).toContain('git rev-list --count origin/<default> --since="<window>"');
+  });
 });
 
 // --- Plan status footer in preamble ---