diff --git a/harness/temporalHelpers.js b/harness/temporalHelpers.js
index 3a2ed5e4d45..6a439b0e8fd 100644
--- a/harness/temporalHelpers.js
+++ b/harness/temporalHelpers.js
@@ -1312,10 +1312,10 @@ var TemporalHelpers = {
         if (this._shiftNanoseconds > 0) {
           if (this._isBeforeShift(instant)) return [instant];
           if (instant.epochNanoseconds < this._epoch2) return [];
-          return [instant.add(this._shift)];
+          return [instant.subtract(this._shift)];
         }
         if (instant.epochNanoseconds < this._epoch2) return [instant];
-        const shifted = instant.add(this._shift);
+        const shifted = instant.subtract(this._shift);
         if (this._isBeforeShift(instant)) return [instant, shifted];
         return [shifted];
       }
diff --git a/test/harness/temporalHelpers-one-shift-time-zone.js b/test/harness/temporalHelpers-one-shift-time-zone.js
new file mode 100644
index 00000000000..db4e4102d0a
--- /dev/null
+++ b/test/harness/temporalHelpers-one-shift-time-zone.js
@@ -0,0 +1,80 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: >
+  Verify the time zone arithmetic used in TemporalHelpers.oneShiftTimeZone()
+  against known cases in the implementation's time zone database
+includes: [compareArray.js, temporalHelpers.js]
+features: [Temporal]
+---*/
+
+function checkTimeZoneArithmetic(shiftInstant, shiftNs, realTimeZoneName, shiftWallTime) {
+  // No need to test this on hosts that don't provide an Intl object. It's
+  // sufficient that the logic is tested on at least one host.
+  if (typeof globalThis.Intl === "undefined")
+    return;
+
+  const tz = TemporalHelpers.oneShiftTimeZone(shiftInstant, shiftNs);
+  const realTz = new Temporal.TimeZone(realTimeZoneName);
+
+  assert.sameValue(
+    tz.getOffsetNanosecondsFor(shiftInstant),
+    realTz.getOffsetNanosecondsFor(shiftInstant),
+    'offset at shift instant'
+  );
+  const minus1 = shiftInstant.subtract({ hours: 1 });
+  assert.sameValue(
+    tz.getOffsetNanosecondsFor(minus1),
+    realTz.getOffsetNanosecondsFor(minus1),
+    'offset at 1 hour before shift'
+  );
+  const plus1 = shiftInstant.add({ hours: 1 });
+  assert.sameValue(
+    tz.getOffsetNanosecondsFor(plus1),
+    realTz.getOffsetNanosecondsFor(plus1),
+    'offset at 1 hour after shift'
+  );
+
+  assert.compareArray(
+    tz.getPossibleInstantsFor(shiftWallTime).map((i) => i.epochNanoseconds),
+    realTz.getPossibleInstantsFor(shiftWallTime).map((i) => i.epochNanoseconds),
+    'possible instants for wall time'
+  );
+  const before1 = shiftWallTime.subtract({ hours: 1 });
+  assert.compareArray(
+    tz.getPossibleInstantsFor(before1).map((i) => i.epochNanoseconds),
+    realTz.getPossibleInstantsFor(before1).map((i) => i.epochNanoseconds),
+    'possible instants for 1 hour before wall time'
+  );
+  const after1 = shiftWallTime.add({ hours: 1 });
+  assert.compareArray(
+    tz.getPossibleInstantsFor(after1).map((i) => i.epochNanoseconds),
+    realTz.getPossibleInstantsFor(after1).map((i) => i.epochNanoseconds),
+    'possible instants for 1 hour after wall time'
+  );
+}
+
+// Check a positive DST shift from +00:00 to +01:00
+checkTimeZoneArithmetic(
+  new Temporal.Instant(1616893200000000000n),
+  3600e9,
+  'Europe/London',
+  new Temporal.PlainDateTime(2021, 3, 28, 1)
+);
+
+// Check a negative DST shift from +00:00 to -01:00
+checkTimeZoneArithmetic(
+  new Temporal.Instant(1635642000000000000n),
+  -3600e9,
+  'Atlantic/Azores',
+  new Temporal.PlainDateTime(2021, 10, 31, 1)
+);
+
+// Check the no-shift case
+checkTimeZoneArithmetic(
+  new Temporal.Instant(0n),
+  0,
+  'UTC',
+  new Temporal.PlainDateTime(1970, 1, 1)
+);