diff --git a/lib/lua/runtime.sx b/lib/lua/runtime.sx
index 99a49521..b23d721b 100644
--- a/lib/lua/runtime.sx
+++ b/lib/lua/runtime.sx
@@ -1238,3 +1238,80 @@
 (define print lua-print)
 (define tostring lua-tostring)
 (define tonumber lua-tonumber)
+
+;; ── os library (minimal stub — no real clock/filesystem) ──────
+(define os {})
+(define __os-counter 0)
+
+(define
+  lua-os-time
+  (fn (&rest args)
+    (begin
+      (set! __os-counter (+ __os-counter 1))
+      __os-counter)))
+
+(define
+  lua-os-clock
+  (fn ()
+    (/ __os-counter 1000)))
+
+(define
+  lua-os-difftime
+  (fn (t2 t1) (- t2 t1)))
+
+(define
+  lua-os-date
+  (fn (&rest args)
+    (let ((fmt (if (> (len args) 0) (first args) "%c")))
+      (cond
+        ((= fmt "*t")
+          (let ((d {}))
+            (begin
+              (dict-set! d "year" 1970)
+              (dict-set! d "month" 1)
+              (dict-set! d "day" 1)
+              (dict-set! d "hour" 0)
+              (dict-set! d "min" 0)
+              (dict-set! d "sec" 0)
+              (dict-set! d "wday" 5)
+              (dict-set! d "yday" 1)
+              (dict-set! d "isdst" false)
+              d)))
+        (else "1970-01-01 00:00:00")))))
+
+(define
+  lua-os-getenv
+  (fn (name) nil))
+
+(define
+  lua-os-exit
+  (fn (&rest args) (error "lua: os.exit called")))
+
+(define
+  lua-os-remove
+  (fn (name)
+    (list (quote lua-multi) nil "os.remove not supported")))
+
+(define
+  lua-os-rename
+  (fn (a b)
+    (list (quote lua-multi) nil "os.rename not supported")))
+
+(define
+  lua-os-tmpname
+  (fn () "/tmp/lua_stub"))
+
+(define
+  lua-os-execute
+  (fn (&rest args) 0))
+
+(dict-set! os "time" lua-os-time)
+(dict-set! os "clock" lua-os-clock)
+(dict-set! os "difftime" lua-os-difftime)
+(dict-set! os "date" lua-os-date)
+(dict-set! os "getenv" lua-os-getenv)
+(dict-set! os "exit" lua-os-exit)
+(dict-set! os "remove" lua-os-remove)
+(dict-set! os "rename" lua-os-rename)
+(dict-set! os "tmpname" lua-os-tmpname)
+(dict-set! os "execute" lua-os-execute)
diff --git a/lib/lua/test.sh b/lib/lua/test.sh
index 92ca2d5f..acc0d006 100755
--- a/lib/lua/test.sh
+++ b/lib/lua/test.sh
@@ -858,6 +858,24 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 1441)
 (eval "(lua-eval-ast \"if io.open(\\\"x\\\") == nil then return 1 else return 0 end\")")
 
+;; ── Phase 6: os stub ──────────────────────────────────────────
+(epoch 1500)
+(eval "(lua-eval-ast \"return type(os.time())\")")
+(epoch 1501)
+(eval "(lua-eval-ast \"local t1 = os.time() local t2 = os.time() return t2 > t1\")")
+(epoch 1502)
+(eval "(lua-eval-ast \"return os.difftime(100, 60)\")")
+(epoch 1503)
+(eval "(lua-eval-ast \"return type(os.clock())\")")
+(epoch 1504)
+(eval "(lua-eval-ast \"return type(os.date())\")")
+(epoch 1505)
+(eval "(lua-eval-ast \"local d = os.date(\\\"*t\\\") return d.year\")")
+(epoch 1506)
+(eval "(lua-eval-ast \"if os.getenv(\\\"HOME\\\") == nil then return 1 else return 0 end\")")
+(epoch 1507)
+(eval "(lua-eval-ast \"return type(os.tmpname())\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 60 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1289,6 +1307,16 @@ check 1431 "tonumber(\"abc\") → nil"        '1'
 check 1440 "io.read() → nil"                '1'
 check 1441 "io.open(x) → nil"               '1'
 
+# ── Phase 6: os stub ──────────────────────────────────────────
+check 1500 "os.time → number"               '"number"'
+check 1501 "os.time monotonic"              'true'
+check 1502 "os.difftime"                    '40'
+check 1503 "os.clock → number"              '"number"'
+check 1504 "os.date() default"              '"string"'
+check 1505 "os.date(*t).year"               '1970'
+check 1506 "os.getenv → nil"                '1'
+check 1507 "os.tmpname → string"            '"string"'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL Lua-on-SX tests passed"
diff --git a/plans/lua-on-sx.md b/plans/lua-on-sx.md
index ff6602de..0eeaaf93 100644
--- a/plans/lua-on-sx.md
+++ b/plans/lua-on-sx.md
@@ -72,7 +72,7 @@ Each item: implement → tests → tick box → update progress log.
 - [x] `math` — full surface
 - [x] `table` — `insert`, `remove`, `concat`, `sort`, `unpack`
 - [x] `io` — minimal stub (read/write to SX IO surface)
-- [ ] `os` — time/date subset
+- [x] `os` — time/date subset
 
 ### Phase 7 — modules + full conformance
 - [ ] `require` / `package` via SX `define-library`/`import`
@@ -82,6 +82,7 @@ Each item: implement → tests → tick box → update progress log.
 
 _Newest first. Agent appends on every commit._
 
+- 2026-04-24: lua: `os` stub — time/clock monotonic counter, difftime, date (default string / `*t` dict), getenv/remove/rename/tmpname/execute/exit stubs. Phase 6 complete. 342 tests.
 - 2026-04-24: lua: `io` stub + `print`/`tostring`/`tonumber` globals. io buffers to internal `__io-buffer` (tests drain it via `io.__buffer()`). print: tab-sep + NL. tostring respects `__tostring` metamethod. 334 tests.
 - 2026-04-24: lua: `table` lib — insert (append / at pos, shifts up), remove (last / at pos, shifts down), concat (sep, i, j), sort (insertion sort, optional cmp), unpack + table.unpack, maxn. Caught trap: local helper named `shift` collides with SX's `shift` special form → renamed to `tbl-shift-up`/`tbl-shift-down`. 322 tests.
 - 2026-04-24: lua: `math` lib — pi/huge + abs/ceil/floor/sqrt/exp/log/log10/pow/trig (sin/cos/tan/asin/acos/atan/atan2)/deg/rad/min/max (&rest)/fmod/modf/random (0/1/2 arg)/randomseed. Most ops delegate to SX primitives; log w/ base via change-of-base. 309 tests.
@@ -109,6 +110,7 @@ _Shared-file issues that need someone else to fix. Minimal repro only._
 
 ## Known limitations (own code, not shared)
 
+- **`os` library is a stub** — `os.time()` returns a monotonic counter (not Unix epoch), `os.clock()` = counter/1000, `os.date()` returns hardcoded "1970-01-01 00:00:00" or a `*t` table with fixed fields; `os.getenv` returns nil; `os.remove`/`rename` return nil+error. No real clock/filesystem access.
 - **`io` library is a stub** — `io.write`/`print` append to an internal `__io-buffer` (accessible via `io.__buffer()` which returns + clears it) instead of real stdout. `io.read`/`open`/`lines` return nil. Suitable for tests that inspect output; no actual stdio.
 - **`string.find`/`match`/`gmatch`/`gsub` patterns are LITERAL only** — no `%d`/`%a`/`.`/`*`/`+`/etc. Implementing Lua patterns is a separate work item; literal search covers the common case.
 - **`string.format`** supports only `%s`, `%d`, `%f`, `%%`. No width/precision flags (`%.2f`, `%5d`).