diff --git a/PR.md b/PR.md
new file mode 100644
index 0000000..a03554d
--- /dev/null
+++ b/PR.md
@@ -0,0 +1,79 @@
+# Fix hour() setter clamping to an invalid hour (24)
+
+## Problem
+
+Calling `.hour(n)` with `n >= 24` produces an invalid spacetime whose epoch is
+`null`, instead of clamping to a valid hour:
+
+```js
+const s = spacetime('2021-03-14 10:00:00', 'UTC')
+
+s.hour(24).isValid()      // false
+s.hour(24).epoch          // null
+s.hour(25).format('iso')  // '' (empty - object is invalid)
+```
+
+This is inconsistent with every other overflow-clamping setter:
+
+- `.date(40)` clamps to the last day of the month,
+- `.month(13)` clamps to December (11),
+- `.hour(-1)` clamps to 0.
+
+Only the upper bound of `.hour()` is broken.
+
+## Root cause
+
+`src/methods/set/set.js`, in `hours()`:
+
+```js
+if (n >= 24) {
+  n = 24
+} else if (n < 0) {
+  n = 0
+}
+```
+
+Valid hours are `0-23`. Clamping to `24` yields a value that
+`src/methods/set/walk.js` rejects (`hour.valid: (n) => n >= 0 && n < 24`), and
+on an invalid unit `walkTo()` sets `s.epoch = null`, invalidating the whole
+object. The negative branch is already correct (clamps to the valid `0`); the
+`>= 24` branch is off by one and should clamp to `23`.
+
+## Fix
+
+```js
+if (n >= 24) {
+  n = 23
+} else if (n < 0) {
+  n = 0
+}
+```
+
+`hour(24)`, `hour(25)`, `hour(100)` now all clamp to `23:00` on the same day,
+matching the behavior of the other setters.
+
+## Test
+
+Added a test to `test/set.test.js` (next to the existing month/date clamp test)
+asserting that out-of-range hours clamp to `23`, keep the object valid, keep the
+same day, and that negatives still clamp to `0`.
+
+Counterfactual (revert `n = 23` back to `n = 24`):
+
+```
+not ok - hour(24) clamps to 23
+  operator: equal
+  expected: 23
+  actual:   0
+```
+
+(the invalid object reports hour `0`).
+
+## Gate
+
+- `npm test` (tape) run against `src` (the default dev target of
+  `test/lib/index.js`): the new test passes; the full suite shows only 18
+  pre-existing DST/timezone failures (`europe/london`, `atlantic/madeira`,
+  `atlantic/azores`, some date-dependent "dst-now") that are present verbatim
+  without this change and are unrelated to it.
+- `npm run lint` (eslint ./src): exit 0.
diff --git a/src/methods/set/set.js b/src/methods/set/set.js
index 1c7d087..33e83c8 100644
--- a/src/methods/set/set.js
+++ b/src/methods/set/set.js
@@ -67,8 +67,10 @@ const minutes = function (s, n, goFwd) {
 
 const hours = function (s, n, goFwd) {
   n = validate(n)
+  // clamp to a valid hour (0-23). Clamping to 24 produces an hour that
+  // walkTo() rejects, which nulls the epoch and invalidates the object.
   if (n >= 24) {
-    n = 24
+    n = 23
   } else if (n < 0) {
     n = 0
   }
diff --git a/test/set.test.js b/test/set.test.js
index 9efdcee..8acbaa2 100644
--- a/test/set.test.js
+++ b/test/set.test.js
@@ -238,3 +238,17 @@ test('month()/date() setters clamp the day to the month length', (t) => {
   t.equal(iso(spacetime('2021-02-10').date(31)), '2021-02-28', 'date(31) in a common february clamps to the 28th')
   t.end()
 })
+
+test('hour() setter clamps out-of-range hours to a valid hour', (t) => {
+  const s = spacetime('2021-03-14 10:00:00', 'UTC')
+  // an out-of-range hour must clamp to the last valid hour (23), not produce
+  // an invalid spacetime with a null epoch
+  t.equal(s.hour(24).hour(), 23, 'hour(24) clamps to 23')
+  t.equal(s.hour(25).hour(), 23, 'hour(25) clamps to 23')
+  t.equal(s.hour(100).hour(), 23, 'hour(100) clamps to 23')
+  t.equal(s.hour(24).isValid(), true, 'hour(24) stays valid')
+  t.equal(s.hour(24).format('iso-short'), '2021-03-14', 'hour(24) keeps the same day')
+  // negative hours still clamp to 0
+  t.equal(s.hour(-1).hour(), 0, 'hour(-1) clamps to 0')
+  t.end()
+})