diff --git a/lib/commerce/conformance.sh b/lib/commerce/conformance.sh
index 161a4a64..0a0bd7cf 100755
--- a/lib/commerce/conformance.sh
+++ b/lib/commerce/conformance.sh
@@ -17,7 +17,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(catalog cart price api promo stack quote ledger order recon federation attribution payment)
+SUITES=(catalog cart price api promo stack quote ledger order recon federation attribution payment window)
 
 OUT_JSON="lib/commerce/scoreboard.json"
 OUT_MD="lib/commerce/scoreboard.md"
@@ -66,6 +66,7 @@ run_suite() {
 (load "lib/commerce/promo.sx")
 (load "lib/commerce/stack.sx")
 (load "lib/commerce/quote.sx")
+(load "lib/commerce/window.sx")
 (load "lib/commerce/ledger.sx")
 (load "lib/commerce/order.sx")
 (load "lib/commerce/payment.sx")
diff --git a/lib/commerce/scoreboard.json b/lib/commerce/scoreboard.json
index eb9fa90f..130ac344 100644
--- a/lib/commerce/scoreboard.json
+++ b/lib/commerce/scoreboard.json
@@ -12,9 +12,10 @@
     "recon": {"pass": 20, "fail": 0},
     "federation": {"pass": 12, "fail": 0},
     "attribution": {"pass": 16, "fail": 0},
-    "payment": {"pass": 7, "fail": 0}
+    "payment": {"pass": 7, "fail": 0},
+    "window": {"pass": 19, "fail": 0}
   },
-  "total_pass": 209,
+  "total_pass": 228,
   "total_fail": 0,
-  "total": 209
+  "total": 228
 }
diff --git a/lib/commerce/scoreboard.md b/lib/commerce/scoreboard.md
index ec38636c..5db4004c 100644
--- a/lib/commerce/scoreboard.md
+++ b/lib/commerce/scoreboard.md
@@ -17,4 +17,5 @@ _Generated by `lib/commerce/conformance.sh`_
 | federation | 12 | 0 | 12 |
 | attribution | 16 | 0 | 16 |
 | payment | 7 | 0 | 7 |
-| **Total** | **209** | **0** | **209** |
+| window | 19 | 0 | 19 |
+| **Total** | **228** | **0** | **228** |
diff --git a/lib/commerce/tests/window.sx b/lib/commerce/tests/window.sx
new file mode 100644
index 00000000..dd3e65a8
--- /dev/null
+++ b/lib/commerce/tests/window.sx
@@ -0,0 +1,112 @@
+;; lib/commerce/tests/window.sx — time-windowed promotions.
+;; Uses (commerce-test name got expected) provided by conformance.sh.
+
+(define
+  pcat
+  (make-catalog (list (list "widget" 1000 :standard)) (list) (list)))
+
+(define gctx (make-pricing-context pcat (list) :uk :guest))
+(define cart (list (list "widget" :none 3)))
+
+(define ten (list :percent "TEN" :standard 1000))
+(define twenty (list :percent "TWENTY" :standard 2000))
+(define always (list :fixed "ALWAYS" 0 100))
+
+(define
+  windowed
+  (list
+    (windowed-promo ten 100 200)
+    (windowed-promo twenty 150 300)
+    (windowed-promo always nil nil)))
+
+(define exclusions (list (list "TEN" "TWENTY")))
+
+;; --- wp-active? boundaries (inclusive) ---
+
+(commerce-test
+  "active-at-from"
+  (wp-active? (windowed-promo ten 100 200) 100)
+  true)
+(commerce-test
+  "active-at-until"
+  (wp-active? (windowed-promo ten 100 200) 200)
+  true)
+(commerce-test
+  "inactive-before"
+  (wp-active? (windowed-promo ten 100 200) 99)
+  false)
+(commerce-test
+  "inactive-after"
+  (wp-active? (windowed-promo ten 100 200) 201)
+  false)
+(commerce-test
+  "open-ended-always"
+  (wp-active? (windowed-promo always nil nil) 99999)
+  true)
+(commerce-test
+  "open-lower"
+  (wp-active? (windowed-promo ten nil 200) 1)
+  true)
+(commerce-test
+  "open-upper"
+  (wp-active? (windowed-promo ten 100 nil) 99999)
+  true)
+
+;; --- active-ruleset filtering ---
+
+(commerce-test
+  "active-ruleset-120"
+  (active-ruleset windowed 120)
+  (list ten always))
+(commerce-test
+  "active-ruleset-160"
+  (active-ruleset windowed 160)
+  (list ten twenty always))
+(commerce-test
+  "active-ruleset-250"
+  (active-ruleset windowed 250)
+  (list twenty always))
+(commerce-test
+  "active-ruleset-50"
+  (active-ruleset windowed 50)
+  (list always))
+
+;; --- active-codes (backward query) ---
+
+(commerce-test
+  "active-codes-120"
+  (active-codes windowed 120)
+  (list "TEN" "ALWAYS"))
+(commerce-test
+  "active-codes-160"
+  (active-codes windowed 160)
+  (list "TEN" "TWENTY" "ALWAYS"))
+(commerce-test
+  "active-codes-50"
+  (active-codes windowed 50)
+  (list "ALWAYS"))
+
+;; --- windowed-quote: discount changes with time (deterministic) ---
+;; subtotal 3000, no tax. TEN=300, TWENTY=600, ALWAYS=100; TEN/TWENTY exclusive.
+
+(commerce-test
+  "quote-50"
+  (quote-discount (windowed-quote gctx cart windowed exclusions 50))
+  100)
+(commerce-test
+  "quote-120"
+  (quote-discount (windowed-quote gctx cart windowed exclusions 120))
+  400)
+(commerce-test
+  "quote-160"
+  (quote-discount (windowed-quote gctx cart windowed exclusions 160))
+  700)
+(commerce-test
+  "quote-250"
+  (quote-discount (windowed-quote gctx cart windowed exclusions 250))
+  700)
+
+(commerce-test
+  "quote-total-160"
+  (quote-total (windowed-quote gctx cart windowed exclusions 160))
+  2300)
diff --git a/lib/commerce/window.sx b/lib/commerce/window.sx
new file mode 100644
index 00000000..461ee089
--- /dev/null
+++ b/lib/commerce/window.sx
@@ -0,0 +1,55 @@
+;; lib/commerce/window.sx — time-windowed promotions.
+;;
+;; A promo's validity window is kept SEPARATE from the promo tuple (so promo.sx
+;; is untouched): a windowed promo is (list promo from until) with inclusive
+;; integer timestamps (same time model as the ledger `at`). nil from = no lower
+;; bound; nil until = open-ended.
+;;
+;; `active-ruleset` filters a windowed ruleset to the plain promos live at a
+;; given time, which feeds straight into promo/stack/quote — so a datetime-aware
+;; quote is just the existing pipeline over the active set. Deterministic: the
+;; quote is a pure function of (ctx, cart, windowed-ruleset, exclusions, at).
+
+(define windowed-promo (fn (promo from until) (list promo from until)))
+
+(define wp-promo (fn (wp) (nth wp 0)))
+(define wp-from (fn (wp) (nth wp 1)))
+(define wp-until (fn (wp) (nth wp 2)))
+
+(define
+  wp-active?
+  (fn
+    (wp at)
+    (let
+      ((from (wp-from wp)) (until (wp-until wp)))
+      (and (or (nil? from) (>= at from)) (or (nil? until) (<= at until))))))
+
+;; Plain promo tuples live at time `at` — feed into cart-quote / best-promo-*.
+(define
+  active-ruleset
+  (fn
+    (windowed at)
+    (map wp-promo (filter (fn (wp) (wp-active? wp at)) windowed))))
+
+;; Relation: which promo codes are active at `at`? (backward query)
+(define
+  active-promoo
+  (fn
+    (windowed at code)
+    (fresh
+      (wp)
+      (membero wp windowed)
+      (project
+        (wp)
+        (if (wp-active? wp at) (== code (promo-code (wp-promo wp))) fail)))))
+
+(define
+  active-codes
+  (fn (windowed at) (run* code (active-promoo windowed at code))))
+
+;; Datetime-aware quote: the existing pipeline over the time-active ruleset.
+(define
+  windowed-quote
+  (fn
+    (ctx cart windowed exclusions at)
+    (cart-quote ctx cart (active-ruleset windowed at) exclusions)))
diff --git a/plans/commerce-on-sx.md b/plans/commerce-on-sx.md
index e4cdf920..f344dc0a 100644
--- a/plans/commerce-on-sx.md
+++ b/plans/commerce-on-sx.md
@@ -21,7 +21,7 @@ reconciliation — all auditable via the event log.
 
 ## Status (rolling)
 
-`bash lib/commerce/conformance.sh` → **209/209** (13 suites; + payment) — **roadmap complete; Phase 5 extensions in progress**
+`bash lib/commerce/conformance.sh` → **228/228** (14 suites; + window) — **roadmap complete; Phase 5 extensions in progress**
 
 ## Ground rules
 
@@ -81,8 +81,9 @@ that unlocks the most tests per effort each iteration.
 - [x] line-level discount attribution — "which line item triggered this discount?"
   as a backward miniKanren query (`attribution.sx`: `promo-toucheso` relation,
   `lines-for-code`/`codes-for-line` both directions, `order-level-codes` for fixed).
-- [ ] time-windowed promotions — promos gated by a validity window; quote takes a
-  datetime, determinism preserved. (quote.sx already documents datetime intent.)
+- [x] time-windowed promotions — `window.sx`: windowed promo `(promo from until)`,
+  `active-ruleset`/`active-codes`/`windowed-quote` gate by datetime; feeds the
+  existing promo/stack/quote pipeline unchanged. Determinism preserved.
 - [ ] discount-aware tax policy — alternative `cart-quote` computing tax on the
   net (post-discount) base via proportional class allocation; explicit + tested.
 - [ ] refund as a flow — refund lifecycle (request → approve → settle) as a second
@@ -96,6 +97,13 @@ that unlocks the most tests per effort each iteration.
   agnostic; `order-settle!(ref, amount)` is the resume seam.
 
 ## Progress log
+- 2026-06-07 — `window.sx` (Phase 5 ext): time-windowed promotions. A validity
+  window is kept SEPARATE from the promo tuple — windowed promo `(promo from
+  until)` (inclusive int timestamps, nil = open bound). `active-ruleset` filters
+  to the plain promos live at `at` and feeds the existing promo/stack/quote
+  pipeline unchanged (promo.sx untouched); `active-promoo`/`active-codes` is the
+  backward "which codes are live at T?" query; `windowed-quote` is the
+  datetime-aware quote (deterministic in `at`). window suite 19/19; total 228/228.
 - 2026-06-07 — `payment.sx` (Phase 5 ext, the item the user asked about):
   provider-neutral payment-request envelope, materialised at the IO edge from the
   ledger amount + host-supplied currency/return-url — keeps lib/commerce vendor-