diff --git a/packages/runtime/src/scr_lib.c b/packages/runtime/src/scr_lib.c
index a9fd25915..1a625cdf1 100644
--- a/packages/runtime/src/scr_lib.c
+++ b/packages/runtime/src/scr_lib.c
@@ -11588,8 +11588,8 @@ double scr_date_get_timezone_offset(double ms) {
  * from local fields reads those fields back. */
 
 /* The host zone's offset (local - UTC) in ms at UTC instant `ms` (an
- * integral time value within about ±8.65e15). */
-static bool scr_date_offset_at(double ms, double *out) {
+ * integral time value within about ±8.65e15), from the host's localtime. */
+static bool scr_date_offset_at_uncached(double ms, double *out) {
   long long cms = (long long)ms;
   double secd = (double)(cms / 1000 - (cms % 1000 < 0));
   struct tm tmv;
@@ -11611,6 +11611,60 @@ static bool scr_date_offset_at(double ms, double *out) {
   return true;
 }
 
+/* Offsets at UTC midnights, for scr_date_offset_at. A day whose two
+ * midnights have the same offset has that offset throughout, given at most
+ * one transition per day (scr_date_local_to_utc already takes transitions
+ * to be at least a day apart). Each local setter asks for three to five
+ * offsets around one time value, and consecutive ones land on the same or
+ * neighbouring days, so most answers come from here instead of a
+ * localtime(3) call: glibc takes its zone lock in each and, for dates past
+ * the zone file's last transition, parses the zone's rule string again. A
+ * change of process.env.TZ starts a new generation. */
+#define SCR_DATE_MIDNIGHTS 64
+typedef struct {
+  long long day;
+  double offset;
+  bool valid;
+} ScrDateMidnight;
+typedef struct {
+  bool valid;
+  unsigned generation;
+  ScrDateMidnight entries[SCR_DATE_MIDNIGHTS];
+} ScrDateMidnightCache;
+static SCR_TL ScrDateMidnightCache scr_date_midnight_cache;
+
+static bool scr_date_midnight_offset(ScrDateMidnightCache *c, long long day, double *out) {
+  ScrDateMidnight *e = &c->entries[(unsigned long long)day % SCR_DATE_MIDNIGHTS];
+  if (e->valid && e->day == day) {
+    *out = e->offset;
+    return true;
+  }
+  if (!scr_date_offset_at_uncached((double)day * 86400000.0, out)) return false;
+  e->valid = true;
+  e->day = day;
+  e->offset = *out;
+  return true;
+}
+
+static bool scr_date_offset_at(double ms, double *out) {
+  ScrDateMidnightCache *c = &scr_date_midnight_cache;
+  unsigned generation = scr_date_tz_generation;
+  if (!c->valid || c->generation != generation) {
+    memset(c, 0, sizeof *c);
+    c->valid = true;
+    c->generation = generation;
+  }
+  long long cms = (long long)ms;
+  long long day = cms / 86400000 - (cms % 86400000 < 0);
+  double start, end;
+  if (scr_date_midnight_offset(c, day, &start) && scr_date_midnight_offset(c, day + 1, &end) &&
+      start == end) {
+    *out = start;
+    return true;
+  }
+  return scr_date_offset_at_uncached(ms, out);
+}
+
 static volatile const double scr_date_day_ms = 86400000.0;
 
 static double scr_date_local_to_utc(double t) {