diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/5683-date-local-fields-tz.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/5683-date-local-fields-tz.ts.json
new file mode 100644
index 000000000..7ac133650
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/5683-date-local-fields-tz.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/5683-date-local-fields-tz.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/runtime/src/scr_lib.c b/packages/runtime/src/scr_lib.c
index 9307d9b01..fd51e9c78 100644
--- a/packages/runtime/src/scr_lib.c
+++ b/packages/runtime/src/scr_lib.c
@@ -824,6 +824,26 @@ static void scr_node_debug_capture(const ScrStr *name) {
   if (name->len == 10 && memcmp(name->data, "NODE_DEBUG", 10) == 0) (void)scr_node_debug_initial();
 }
 
+/* Counts writes and deletes of process.env.TZ. The local-time getters keep
+ * their last conversion for the generation it was made in. */
+#ifdef SCR_WORKERS
+static atomic_uint scr_date_tz_generation;
+#else
+static unsigned scr_date_tz_generation;
+#endif
+
+/* After a write or delete of the process's TZ, local time follows the new
+ * value, as in Node: tzset(3) rereads it for later localtime calls. */
+static void scr_env_tz_changed(const ScrStr *name) {
+  /* Compared by character: a string literal would join the merged
+   * read-only strings of every program that links this file. */
+  if (name->len != 2 || name->data[0] != 'T' || name->data[1] != 'Z') return;
+#if !defined(_WIN32) && !defined(__wasi__)
+  tzset();
+#endif
+  scr_date_tz_generation++;
+}
+
 /* process.env.NAME = v — setenv(3): later scr_env_get reads and spawned
  * children (posix_spawn inherits environ) observe the write, like Node.
  * Both args borrowed (NUL-terminated ScrStr data); setenv copies. */
@@ -842,6 +862,7 @@ void scr_env_set(const ScrStr *name, const ScrStr *value) {
 #else
   setenv(k, v, 1);
 #endif
+  scr_env_tz_changed(name);
 #ifdef SCR_WORKERS
 done:
 #endif
@@ -861,6 +882,7 @@ void scr_env_unset(const ScrStr *name) {
 #else
   unsetenv(name->data);
 #endif
+  scr_env_tz_changed(name);
 }
 
 static ScrStr *scr_fs_read_file_core(ScrStr *path, bool env_file, size_t *ascii);
@@ -10475,9 +10497,14 @@ typedef struct ScrDateParts {
  * endpoint. The checked wrapper is the public UTC-getter gate; the inner
  * walk accepts those bounded local-time values too. */
 static void scr_date_utc_parts_unchecked(double t, ScrDateParts *out) {
-  double dayd = floor(t / 86400000.0);
-  long long msday = (long long)(t - dayd * 86400000.0);
-  long long z = (long long)dayd + 719468;
+  /* t is an integral number of milliseconds within about 2^53. */
+  long long tms = (long long)t;
+  long long day = tms / 86400000, msday = tms % 86400000;
+  if (msday < 0) {
+    msday += 86400000;
+    day--;
+  }
+  long long z = day + 719468;
   long long era = (z >= 0 ? z : z - 146096) / 146097;
   unsigned long long doe = (unsigned long long)(z - era * 146097);
   unsigned long long yoe = (doe - doe / 1460 + doe / 36524 - doe / 146096) / 365;
@@ -10487,7 +10514,7 @@ static void scr_date_utc_parts_unchecked(double t, ScrDateParts *out) {
   unsigned long long d = doy - (153 * mp + 2) / 5 + 1;
   unsigned long long m = mp < 10 ? mp + 3 : mp - 9;
   if (m <= 2) y++;
-  long long wday = ((long long)dayd + 4) % 7;
+  long long wday = (day + 4) % 7;
   if (wday < 0) wday += 7;
   out->year = y;
   out->month = (int)m - 1;
@@ -10516,10 +10543,39 @@ static bool scr_date_localtime(double secd, struct tm *out) {
 #endif
 }
 
+/* The thread's last local conversion: reading several local fields of one
+ * Date (getFullYear, getMonth, getDate, getHours, ...) converts it once.
+ * A change of process.env.TZ starts a new generation. */
+typedef struct {
+  bool valid;
+  unsigned generation;
+  double ms;
+  ScrDateParts parts;
+} ScrDateLocalCache;
+static SCR_TL ScrDateLocalCache scr_date_local_cache;
+
+static bool scr_date_local_parts_uncached(double clipped, ScrDateParts *out);
+
 static bool scr_date_local_parts(double ms, ScrDateParts *out) {
   if (!isfinite(ms) || fabs(ms) > 8640000000000000.0) return false;
   double clipped = trunc(ms);
-  double secd = floor(clipped / 1000.0);
+  ScrDateLocalCache *c = &scr_date_local_cache;
+  unsigned generation = scr_date_tz_generation;
+  if (c->valid && c->ms == clipped && c->generation == generation) {
+    *out = c->parts;
+    return true;
+  }
+  if (!scr_date_local_parts_uncached(clipped, out)) return false;
+  c->valid = true;
+  c->generation = generation;
+  c->ms = clipped;
+  c->parts = *out;
+  return true;
+}
+
+static bool scr_date_local_parts_uncached(double clipped, ScrDateParts *out) {
+  long long cms = (long long)clipped;
+  double secd = (double)(cms / 1000 - (cms % 1000 < 0));
   struct tm tmv;
   double basis_secd = secd;
   if (!scr_date_localtime(basis_secd, &tmv)) {
diff --git a/tests/corpus/5683-date-local-fields-tz.ts b/tests/corpus/5683-date-local-fields-tz.ts
new file mode 100644
index 000000000..dc75011ed
--- /dev/null
+++ b/tests/corpus/5683-date-local-fields-tz.ts
@@ -0,0 +1,95 @@
+// Local-time getters read repeatedly on one Date and alternately on two,
+// on Dates parsed from strings, and across process.env.TZ writes and
+// deletes: every read answers the fields of the zone in effect at that
+// moment, as in Node.
+
+function fields(d: Date): string {
+  return [
+    d.getFullYear(),
+    d.getMonth(),
+    d.getDate(),
+    d.getDay(),
+    d.getHours(),
+    d.getMinutes(),
+    d.getSeconds(),
+    d.getMilliseconds(),
+    d.getTimezoneOffset(),
+  ].join(",");
+}
+
+const a = new Date(Date.UTC(2024, 2, 10, 6, 59, 59, 999));
+const b = new Date(Date.UTC(2024, 2, 10, 7, 0, 0, 0));
+const c = new Date(Date.UTC(2024, 10, 3, 5, 30, 0, 250));
+
+function report(label: string): void {
+  console.log(label, fields(a), fields(a));
+  console.log(label, fields(b), fields(c), fields(b));
+  let mixed = 0;
+  for (let i = 0; i < 6; i++) mixed += a.getHours() * 100 + b.getHours() + c.getDate();
+  console.log(label, "alternating", mixed);
+}
+
+report("start");
+process.env.TZ = "America/New_York";
+report("new york");
+process.env.TZ = "Asia/Kolkata";
+report("kolkata");
+process.env.TZ = "Australia/Lord_Howe";
+report("lord howe");
+
+// Dates parsed from strings with an explicit offset, read in local time.
+const d = new Date("2024-06-01T23:45:00+05:30");
+console.log("offset string", fields(d), d.getTime());
+const e = new Date("2024-07-11T00:15:30.001Z");
+console.log("offset string", fields(e), e.getTime());
+console.log("first again", fields(d));
+
+delete process.env.TZ;
+report("deleted");
+process.env.TZ = "UTC";
+report("utc");
+
+// Many instants read field by field.
+process.env.TZ = "Europe/Berlin";
+let sum = 0;
+for (let i = 0; i < 2000; i++) {
+  const t = new Date(1700000000000 + i * 3_599_999);
+  sum += t.getHours() + t.getDate() * 3 + t.getMonth() * 7 + t.getTimezoneOffset();
+}
+console.log("berlin sum", sum);
+
+const invalid = new Date(Number.NaN);
+console.log("invalid", fields(invalid), fields(invalid));
+
+// Instants around day and second boundaries, before the epoch and at the
+// ends of the Date range, in UTC and local fields; then 2000-2030 in a zone
+// with DST.
+function utcFields(d: Date): string {
+  return [
+    d.getUTCFullYear(),
+    d.getUTCMonth(),
+    d.getUTCDate(),
+    d.getUTCDay(),
+    d.getUTCHours(),
+    d.getUTCMinutes(),
+    d.getUTCSeconds(),
+    d.getUTCMilliseconds(),
+  ].join(",");
+}
+const edges = [
+  0, -0, -0.5, 0.5, -1, 1, -999, -1000, -1001, 999.9, -86_400_000, -86_400_001, 86_399_999,
+  -62_198_755_200_000, -62_198_755_200_001, 8_640_000_000_000_000, -8_640_000_000_000_000,
+  8_639_999_999_999_999, -8_639_999_999_999_999, 951_782_400_000, 4_107_542_399_999,
+];
+process.env.TZ = "UTC";
+for (const t of edges) {
+  const x = new Date(t);
+  console.log(t, utcFields(x), fields(x));
+}
+process.env.TZ = "America/Sao_Paulo";
+let walk = 0;
+for (let t = 946_684_800_000; t < 1_893_456_000_000; t += 77_777_777) {
+  const x = new Date(t);
+  walk = (walk * 31 + x.getUTCDate() + x.getUTCHours() * 7 + x.getDate() + x.getMinutes() + x.getDay()) % 1_000_003;
+}
+console.log("walk", walk);
diff --git a/tests/harness/corpus-costs.json b/tests/harness/corpus-costs.json
index 3fcd101e0..acaa95479 100644
--- a/tests/harness/corpus-costs.json
+++ b/tests/harness/corpus-costs.json
@@ -1997,6 +1997,7 @@
     "5680-array-growth-after-copy.ts": 3791,
     "5681-array-pop-and-length.ts": 3754,
     "5682-publish-wide-objects.ts": 3326,
+    "5683-date-local-fields-tz.ts": 3848,
     "5700-cycle-release-retained.ts": 2156,
     "5701-cycle-release-old-edges.ts": 2601,
     "5702-publish-candidate-buffers.ts": 3469,
@@ -5026,6 +5027,7 @@
     "5680-array-growth-after-copy.ts": 4581,
     "5681-array-pop-and-length.ts": 3216,
     "5682-publish-wide-objects.ts": 3376,
+    "5683-date-local-fields-tz.ts": 3911,
     "5700-cycle-release-retained.ts": 3414,
     "5701-cycle-release-old-edges.ts": 4210,
     "5702-publish-candidate-buffers.ts": 3848,