diff --git a/polyfill/lib/intl.mjs b/polyfill/lib/intl.mjs
index 0193050fa..a4bb2177c 100644
--- a/polyfill/lib/intl.mjs
+++ b/polyfill/lib/intl.mjs
@@ -8,6 +8,7 @@ import {
   TypeError as TypeErrorCtor,
 
   // class static functions and methods
+  ArrayPrototypeIncludes,
   ArrayPrototypeSlice,
   IntlDateTimeFormat,
   IntlDateTimeFormatPrototypeGetFormat,
@@ -22,6 +23,7 @@ import {
   IntlNumberFormat,
   IntlNumberFormatPrototypeGetFormat,
   MathCeil,
+  NumberIsFinite,
   NumberIsNaN,
   ObjectAssign,
   ObjectCreate,
@@ -131,37 +133,62 @@ function adjustRenderedWeekday(polyfilledFormatter, realFormatter, originalEpoch
 function internalCreateDateTimeFormat(dtf, locale, options, required) {
   const hasOptions = typeof options !== 'undefined';
   if (hasOptions) {
-    // Read all the options in the expected order and copy them to a
-    // null-prototype object with which we can do further operations
-    // unobservably
+    // Read all the options in the expected order, perform expected conversions
+    // and validations, and copy them to a null-prototype object with which we
+    // can do further operations unobservably
     const props = [
-      'localeMatcher',
-      'calendar',
-      'numberingSystem',
-      'hour12',
-      'hourCycle',
-      'timeZone',
-      'weekday',
-      'era',
-      'year',
-      'month',
-      'day',
-      'dayPeriod',
-      'hour',
-      'minute',
-      'second',
-      'fractionalSecondDigits',
-      'timeZoneName',
-      'formatMatcher',
-      'dateStyle',
-      'timeStyle'
+      ['localeMatcher', 'string', ['lookup', 'best fit']],
+      ['calendar', 'string', null],
+      ['numberingSystem', 'string', null],
+      ['hour12', 'boolean'],
+      ['hourCycle', 'string', ['h11', 'h12', 'h23', 'h24']],
+      ['timeZone', 'string', null],
+      ['weekday', 'string', ['narrow', 'short', 'long']],
+      ['era', 'string', ['narrow', 'short', 'long']],
+      ['year', 'string', ['2-digit', 'numeric']],
+      ['month', 'string', ['2-digit', 'numeric', 'narrow', 'short', 'long']],
+      ['day', 'string', ['2-digit', 'numeric']],
+      ['dayPeriod', 'string', ['narrow', 'short', 'long']],
+      ['hour', 'string', ['2-digit', 'numeric']],
+      ['minute', 'string', ['2-digit', 'numeric']],
+      ['second', 'string', ['2-digit', 'numeric']],
+      ['fractionalSecondDigits', 'number', 1, 3],
+      ['timeZoneName', 'string', ['short', 'long', 'shortOffset', 'longOffset', 'shortGeneric', 'longGeneric']],
+      ['formatMatcher', 'string', ['basic', 'best fit']],
+      ['dateStyle', 'string', ['full', 'long', 'medium', 'short']],
+      ['timeStyle', 'string', ['full', 'long', 'medium', 'short']]
     ];
     options = ES.ToObject(options);
     const newOptions = ObjectCreate(null);
     for (let i = 0; i < props.length; i++) {
-      const prop = props[i];
-      if (ES.HasOwnProperty(options, prop)) {
-        newOptions[prop] = options[prop];
+      const entry = props[i];
+      const prop = entry[0];
+      let value = options[prop];
+      if (value !== undefined) {
+        const conversionHint = entry[1];
+        switch (conversionHint) {
+          case 'string':
+            {
+              value = ES.ToString(value);
+              const allowed = entry[2];
+              if (allowed !== null && !ES.Call(ArrayPrototypeIncludes, allowed, [value])) {
+                throw new RangeError(`Value ${value} out of range for Intl.DateTimeFormat options property ${prop}`);
+              }
+            }
+            break;
+          case 'number':
+            {
+              value = ES.ToNumber(value);
+              const minimum = entry[2];
+              const maximum = entry[3];
+              if (!NumberIsFinite(value) || value < minimum || value > maximum) {
+                throw new RangeError(`${prop} value is out of range`);
+              }
+            }
+            break;
+          case 'boolean': // nothing observable, handle in real DTF constructor
+        }
+        newOptions[prop] = value;
       }
     }
     options = newOptions;
diff --git a/polyfill/lib/zoneddatetime.mjs b/polyfill/lib/zoneddatetime.mjs
index 7b82870c0..dda981717 100644
--- a/polyfill/lib/zoneddatetime.mjs
+++ b/polyfill/lib/zoneddatetime.mjs
@@ -287,7 +287,7 @@ export class ZonedDateTime {
 
     // first, round the underlying DateTime fields
     const timeZone = GetSlot(this, TIME_ZONE);
-    const thisNs = GetSlot(this, EPOCHNANOSECONDS);
+    let thisNs = GetSlot(this, EPOCHNANOSECONDS);
     const iso = dateTime(this);
     let epochNanoseconds;
 
@@ -301,7 +301,13 @@ export class ZonedDateTime {
       assert(thisNs.geq(startNs), 'cannot produce an instant during a day that occurs before start-of-day instant');
 
       const endNs = ES.GetStartOfDay(timeZone, dateEnd);
-      assert(thisNs.lt(endNs), 'cannot produce an instant during a day that occurs on or after end-of-day instant');
+      // Handle the case where a transition starts after midnight and falls back
+      // to before midnight, and pieces of two calendar days are interleaved.
+      // endNs is the start of the first piece of the second calendar day, so if
+      // thisNs is inside the second piece of the first calendar day, it can be
+      // outside of the box defined by start-of-day and end-of-day.
+      // https://github.com/tc39/proposal-temporal/issues/3312
+      if (thisNs.geq(endNs)) thisNs = endNs.minus(bigInt.one);
 
       const dayLengthNs = endNs.subtract(startNs);
       const dayProgressNs = TimeDuration.fromEpochNsDiff(thisNs, startNs);
diff --git a/polyfill/test/thorough/startofday.mjs b/polyfill/test/thorough/startofday.mjs
index 33070973c..970b5ef1b 100644
--- a/polyfill/test/thorough/startofday.mjs
+++ b/polyfill/test/thorough/startofday.mjs
@@ -1,4 +1,12 @@
-import { getProgressBar, makeZonedCases, time, withSnapshotsFromFile } from './support.mjs';
+import {
+  assertEqual,
+  assertTemporalEqual,
+  getProgressBar,
+  makeZonedCases,
+  roundingModes,
+  time,
+  withSnapshotsFromFile
+} from './support.mjs';
 
 const interestingCases = makeZonedCases().filter(
   ([datetime]) =>
@@ -13,7 +21,32 @@ await time(async (start) => {
   await withSnapshotsFromFile('./startofday.snapshot.json', (matchSnapshot) => {
     for (const [datetime, str] of interestingCases) {
       progress.tick(1, { test: str });
-      matchSnapshot(datetime.startOfDay().toString(), str);
+      const startToday = datetime.startOfDay();
+      matchSnapshot(startToday.toString(), str);
+
+      // Can't get tomorrow's startOfDay() if this is the last representable day
+      if (datetime.epochMilliseconds > 86400_0000_0000 - 86400) continue;
+
+      const hoursInDay = datetime.hoursInDay;
+      matchSnapshot(hoursInDay, str + 'h');
+
+      // Invariant: startOfDay + hoursInDay = following day's startOfDay
+      const startTomorrow = datetime.add({ days: 1 }).startOfDay();
+      assertTemporalEqual(
+        startToday.add({ seconds: hoursInDay * 3600 }),
+        startTomorrow,
+        `${startToday} + ${hoursInDay}h = ${startTomorrow}`
+      );
+
+      // Invariant: round({ smallestUnit: 'day' }) is start of today or tomorrow
+      for (const roundingMode of roundingModes) {
+        const rounded = datetime.round({ smallestUnit: 'day', roundingMode });
+        assertEqual(
+          rounded.equals(startToday) || rounded.equals(startTomorrow),
+          true,
+          `${rounded} = ${startToday} or ${startTomorrow}`
+        );
+      }
     }
   });
 
diff --git a/polyfill/test/thorough/startofday.snapshot.json b/polyfill/test/thorough/startofday.snapshot.json
index 03e4367cc..db9f77592 100644
--- a/polyfill/test/thorough/startofday.snapshot.json
+++ b/polyfill/test/thorough/startofday.snapshot.json
@@ -10,163 +10,321 @@
   "+275760-09-13T01:23:00+01:23[+01:23]": "+275760-09-13T00:00:00+01:23[+01:23]",
   "+275760-09-13T02:00:00+02:00[Europe/Amsterdam]": "+275760-09-13T00:00:00+02:00[Europe/Amsterdam]",
   "-000001-01-01T00:00:00+00:00[UTC]": "-000001-01-01T00:00:00+00:00[UTC]",
+  "-000001-01-01T00:00:00+00:00[UTC]h": 24,
   "-000001-01-01T00:17:30+00:18[Europe/Amsterdam]": "-000001-01-01T00:00:00+00:18[Europe/Amsterdam]",
+  "-000001-01-01T00:17:30+00:18[Europe/Amsterdam]h": 24,
   "-000001-01-01T01:23:00+01:23[+01:23]": "-000001-01-01T00:00:00+01:23[+01:23]",
+  "-000001-01-01T01:23:00+01:23[+01:23]h": 24,
   "-000002-12-31T11:26:00-12:34[-12:34]": "-000002-12-31T00:00:00-12:34[-12:34]",
+  "-000002-12-31T11:26:00-12:34[-12:34]h": 24,
   "-000002-12-31T15:47:32-08:12[America/Vancouver]": "-000002-12-31T00:00:00-08:12[America/Vancouver]",
+  "-000002-12-31T15:47:32-08:12[America/Vancouver]h": 24,
   "-271821-04-20T00:00:00+00:00[UTC]": "-271821-04-20T00:00:00+00:00[UTC]",
+  "-271821-04-20T00:00:00+00:00[UTC]h": 24,
   "-271821-04-20T11:26:00-12:34[-12:34]": "-271821-04-20T00:00:00-12:34[-12:34]",
+  "-271821-04-20T11:26:00-12:34[-12:34]h": 24,
   "-271821-04-20T15:47:32-08:12[America/Vancouver]": "-271821-04-20T00:00:00-08:12[America/Vancouver]",
+  "-271821-04-20T15:47:32-08:12[America/Vancouver]h": 24,
   "-271821-04-21T00:00:00+00:00[UTC]": "-271821-04-21T00:00:00+00:00[UTC]",
+  "-271821-04-21T00:00:00+00:00[UTC]h": 24,
   "-271821-04-21T00:17:30+00:18[Europe/Amsterdam]": "-271821-04-21T00:00:00+00:18[Europe/Amsterdam]",
+  "-271821-04-21T00:17:30+00:18[Europe/Amsterdam]h": 24,
   "-271821-04-21T01:23:00+01:23[+01:23]": "-271821-04-21T00:00:00+01:23[+01:23]",
+  "-271821-04-21T01:23:00+01:23[+01:23]h": 24,
   "0000-12-31T11:26:00-12:34[-12:34]": "0000-12-31T00:00:00-12:34[-12:34]",
+  "0000-12-31T11:26:00-12:34[-12:34]h": 24,
   "0000-12-31T15:47:32-08:12[America/Vancouver]": "0000-12-31T00:00:00-08:12[America/Vancouver]",
+  "0000-12-31T15:47:32-08:12[America/Vancouver]h": 24,
   "0001-01-01T00:00:00+00:00[UTC]": "0001-01-01T00:00:00+00:00[UTC]",
+  "0001-01-01T00:00:00+00:00[UTC]h": 24,
   "0001-01-01T00:17:30+00:18[Europe/Amsterdam]": "0001-01-01T00:00:00+00:18[Europe/Amsterdam]",
+  "0001-01-01T00:17:30+00:18[Europe/Amsterdam]h": 24,
   "0001-01-01T01:23:00+01:23[+01:23]": "0001-01-01T00:00:00+01:23[+01:23]",
+  "0001-01-01T01:23:00+01:23[+01:23]h": 24,
   "1385-06-11T11:51:26.290448384-12:34[-12:34]": "1385-06-11T00:00:00-12:34[-12:34]",
+  "1385-06-11T11:51:26.290448384-12:34[-12:34]h": 24,
   "1385-06-11T11:51:26.290448385-12:34[-12:34]": "1385-06-11T00:00:00-12:34[-12:34]",
+  "1385-06-11T11:51:26.290448385-12:34[-12:34]h": 24,
   "1385-06-11T16:12:58.290448384-08:12[America/Vancouver]": "1385-06-11T00:00:00-08:12[America/Vancouver]",
+  "1385-06-11T16:12:58.290448384-08:12[America/Vancouver]h": 24,
   "1385-06-11T16:12:58.290448385-08:12[America/Vancouver]": "1385-06-11T00:00:00-08:12[America/Vancouver]",
+  "1385-06-11T16:12:58.290448385-08:12[America/Vancouver]h": 24,
   "1385-06-12T00:25:26.290448384+00:00[UTC]": "1385-06-12T00:00:00+00:00[UTC]",
+  "1385-06-12T00:25:26.290448384+00:00[UTC]h": 24,
   "1385-06-12T00:25:26.290448385+00:00[UTC]": "1385-06-12T00:00:00+00:00[UTC]",
+  "1385-06-12T00:25:26.290448385+00:00[UTC]h": 24,
   "1385-06-12T00:42:56.290448384+00:18[Europe/Amsterdam]": "1385-06-12T00:00:00+00:18[Europe/Amsterdam]",
+  "1385-06-12T00:42:56.290448384+00:18[Europe/Amsterdam]h": 24,
   "1385-06-12T00:42:56.290448385+00:18[Europe/Amsterdam]": "1385-06-12T00:00:00+00:18[Europe/Amsterdam]",
+  "1385-06-12T00:42:56.290448385+00:18[Europe/Amsterdam]h": 24,
   "1385-06-12T01:48:26.290448384+01:23[+01:23]": "1385-06-12T00:00:00+01:23[+01:23]",
+  "1385-06-12T01:48:26.290448384+01:23[+01:23]h": 24,
   "1385-06-12T01:48:26.290448385+01:23[+01:23]": "1385-06-12T00:00:00+01:23[+01:23]",
+  "1385-06-12T01:48:26.290448385+01:23[+01:23]h": 24,
   "1677-09-20T11:38:43.145224191-12:34[-12:34]": "1677-09-20T00:00:00-12:34[-12:34]",
+  "1677-09-20T11:38:43.145224191-12:34[-12:34]h": 24,
   "1677-09-20T11:38:43.145224192-12:34[-12:34]": "1677-09-20T00:00:00-12:34[-12:34]",
+  "1677-09-20T11:38:43.145224192-12:34[-12:34]h": 24,
   "1677-09-20T16:00:15.145224191-08:12[America/Vancouver]": "1677-09-20T00:00:00-08:12[America/Vancouver]",
+  "1677-09-20T16:00:15.145224191-08:12[America/Vancouver]h": 24,
   "1677-09-20T16:00:15.145224192-08:12[America/Vancouver]": "1677-09-20T00:00:00-08:12[America/Vancouver]",
+  "1677-09-20T16:00:15.145224192-08:12[America/Vancouver]h": 24,
   "1677-09-21T00:12:43.145224191+00:00[UTC]": "1677-09-21T00:00:00+00:00[UTC]",
+  "1677-09-21T00:12:43.145224191+00:00[UTC]h": 24,
   "1677-09-21T00:12:43.145224192+00:00[UTC]": "1677-09-21T00:00:00+00:00[UTC]",
+  "1677-09-21T00:12:43.145224192+00:00[UTC]h": 24,
   "1677-09-21T00:30:13.145224191+00:18[Europe/Amsterdam]": "1677-09-21T00:00:00+00:18[Europe/Amsterdam]",
+  "1677-09-21T00:30:13.145224191+00:18[Europe/Amsterdam]h": 24,
   "1677-09-21T00:30:13.145224192+00:18[Europe/Amsterdam]": "1677-09-21T00:00:00+00:18[Europe/Amsterdam]",
+  "1677-09-21T00:30:13.145224192+00:18[Europe/Amsterdam]h": 24,
   "1677-09-21T01:35:43.145224191+01:23[+01:23]": "1677-09-21T00:00:00+01:23[+01:23]",
+  "1677-09-21T01:35:43.145224191+01:23[+01:23]h": 24,
   "1677-09-21T01:35:43.145224192+01:23[+01:23]": "1677-09-21T00:00:00+01:23[+01:23]",
+  "1677-09-21T01:35:43.145224192+01:23[+01:23]h": 24,
   "1901-12-13T08:11:51-12:34[-12:34]": "1901-12-13T00:00:00-12:34[-12:34]",
+  "1901-12-13T08:11:51-12:34[-12:34]h": 24,
   "1901-12-13T08:11:52-12:34[-12:34]": "1901-12-13T00:00:00-12:34[-12:34]",
+  "1901-12-13T08:11:52-12:34[-12:34]h": 24,
   "1901-12-13T12:45:51-08:00[America/Vancouver]": "1901-12-13T00:00:00-08:00[America/Vancouver]",
+  "1901-12-13T12:45:51-08:00[America/Vancouver]h": 24,
   "1901-12-13T12:45:52-08:00[America/Vancouver]": "1901-12-13T00:00:00-08:00[America/Vancouver]",
+  "1901-12-13T12:45:52-08:00[America/Vancouver]h": 24,
   "1901-12-13T20:45:51+00:00[Europe/Amsterdam]": "1901-12-13T00:00:00+00:00[Europe/Amsterdam]",
+  "1901-12-13T20:45:51+00:00[Europe/Amsterdam]h": 24,
   "1901-12-13T20:45:51+00:00[UTC]": "1901-12-13T00:00:00+00:00[UTC]",
+  "1901-12-13T20:45:51+00:00[UTC]h": 24,
   "1901-12-13T20:45:52+00:00[Europe/Amsterdam]": "1901-12-13T00:00:00+00:00[Europe/Amsterdam]",
+  "1901-12-13T20:45:52+00:00[Europe/Amsterdam]h": 24,
   "1901-12-13T20:45:52+00:00[UTC]": "1901-12-13T00:00:00+00:00[UTC]",
+  "1901-12-13T20:45:52+00:00[UTC]h": 24,
   "1901-12-13T22:08:51+01:23[+01:23]": "1901-12-13T00:00:00+01:23[+01:23]",
+  "1901-12-13T22:08:51+01:23[+01:23]h": 24,
   "1901-12-13T22:08:52+01:23[+01:23]": "1901-12-13T00:00:00+01:23[+01:23]",
+  "1901-12-13T22:08:52+01:23[+01:23]h": 24,
   "1919-03-31T12:00:00-04:00[America/Toronto]": "1919-03-31T00:30:00-04:00[America/Toronto]",
+  "1919-03-31T12:00:00-04:00[America/Toronto]h": 23.5,
   "1938-04-24T09:39:20-12:34[-12:34]": "1938-04-24T00:00:00-12:34[-12:34]",
+  "1938-04-24T09:39:20-12:34[-12:34]h": 24,
   "1938-04-24T14:13:20-08:00[America/Vancouver]": "1938-04-24T00:00:00-08:00[America/Vancouver]",
+  "1938-04-24T14:13:20-08:00[America/Vancouver]h": 24,
   "1938-04-24T22:13:20+00:00[UTC]": "1938-04-24T00:00:00+00:00[UTC]",
+  "1938-04-24T22:13:20+00:00[UTC]h": 24,
   "1938-04-24T23:13:20+01:00[Europe/Amsterdam]": "1938-04-24T00:00:00+01:00[Europe/Amsterdam]",
+  "1938-04-24T23:13:20+01:00[Europe/Amsterdam]h": 24,
   "1938-04-24T23:36:20+01:23[+01:23]": "1938-04-24T00:00:00+01:23[+01:23]",
+  "1938-04-24T23:36:20+01:23[+01:23]h": 24,
   "1969-09-18T05:25:59.757604687-12:34[-12:34]": "1969-09-18T00:00:00-12:34[-12:34]",
+  "1969-09-18T05:25:59.757604687-12:34[-12:34]h": 24,
   "1969-09-18T10:59:59.757604687-07:00[America/Vancouver]": "1969-09-18T00:00:00-07:00[America/Vancouver]",
+  "1969-09-18T10:59:59.757604687-07:00[America/Vancouver]h": 24,
   "1969-09-18T17:59:59.757604687+00:00[UTC]": "1969-09-18T00:00:00+00:00[UTC]",
+  "1969-09-18T17:59:59.757604687+00:00[UTC]h": 24,
   "1969-09-18T18:59:59.757604687+01:00[Europe/Amsterdam]": "1969-09-18T00:00:00+01:00[Europe/Amsterdam]",
+  "1969-09-18T18:59:59.757604687+01:00[Europe/Amsterdam]h": 24,
   "1969-09-18T19:22:59.757604687+01:23[+01:23]": "1969-09-18T00:00:00+01:23[+01:23]",
+  "1969-09-18T19:22:59.757604687+01:23[+01:23]h": 24,
   "1969-12-31T11:25:55.705032704-12:34[-12:34]": "1969-12-31T00:00:00-12:34[-12:34]",
+  "1969-12-31T11:25:55.705032704-12:34[-12:34]h": 24,
   "1969-12-31T11:25:55.705032705-12:34[-12:34]": "1969-12-31T00:00:00-12:34[-12:34]",
+  "1969-12-31T11:25:55.705032705-12:34[-12:34]h": 24,
   "1969-12-31T11:25:57.852516351-12:34[-12:34]": "1969-12-31T00:00:00-12:34[-12:34]",
+  "1969-12-31T11:25:57.852516351-12:34[-12:34]h": 24,
   "1969-12-31T11:25:57.852516352-12:34[-12:34]": "1969-12-31T00:00:00-12:34[-12:34]",
+  "1969-12-31T11:25:57.852516352-12:34[-12:34]h": 24,
   "1969-12-31T11:25:59.999999999-12:34[-12:34]": "1969-12-31T00:00:00-12:34[-12:34]",
+  "1969-12-31T11:25:59.999999999-12:34[-12:34]h": 24,
   "1969-12-31T11:26:00-12:34[-12:34]": "1969-12-31T00:00:00-12:34[-12:34]",
+  "1969-12-31T11:26:00-12:34[-12:34]h": 24,
   "1969-12-31T11:26:00.000000001-12:34[-12:34]": "1969-12-31T00:00:00-12:34[-12:34]",
+  "1969-12-31T11:26:00.000000001-12:34[-12:34]h": 24,
   "1969-12-31T11:26:02.147483647-12:34[-12:34]": "1969-12-31T00:00:00-12:34[-12:34]",
+  "1969-12-31T11:26:02.147483647-12:34[-12:34]h": 24,
   "1969-12-31T11:26:02.147483648-12:34[-12:34]": "1969-12-31T00:00:00-12:34[-12:34]",
+  "1969-12-31T11:26:02.147483648-12:34[-12:34]h": 24,
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diff --git a/polyfill/test262 b/polyfill/test262
index d0c1b4555..227d90551 160000
--- a/polyfill/test262
+++ b/polyfill/test262
@@ -1 +1 @@
-Subproject commit d0c1b4555b03dd404873fd6422a4b5da00136500
+Subproject commit 227d905513f790dec90858d04ddf8cf81326706f