diff --git a/test/built-ins/Temporal/Duration/prototype/round/bubble-time-unit.js b/test/built-ins/Temporal/Duration/prototype/round/bubble-time-unit.js
new file mode 100644
index 0000000000..472be62a85
--- /dev/null
+++ b/test/built-ins/Temporal/Duration/prototype/round/bubble-time-unit.js
@@ -0,0 +1,83 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.duration.prototype.round
+description: Rounds relative to a date, bubbling up time units.
+info: |
+  https://github.com/tc39/proposal-temporal/issues/3121
+  Test case that calls BubbleRelativeDuration with a time unit as largestUnit.
+  This happens when time units round up and overflow a date unit, and
+  largestUnit is not "days" or higher, and the rounding is relative to a
+  PlainDateTime (ZonedDateTime would use Instant semantics in that case.)
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const relativeTo = new Temporal.PlainDate(2025, 6, 14);
+
+{
+  const duration = new Temporal.Duration(0, 0, 0, 0, /* hours = */ 14);
+  const result = duration.round({
+    relativeTo,
+    smallestUnit: "hours",
+    roundingIncrement: 12,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 24, 0, 0, 0, 0, 0);
+}
+
+{
+  const duration = new Temporal.Duration(0, 0, 0, 0, 0, /* minutes = */ 1415);
+  const result = duration.round({
+    relativeTo,
+    smallestUnit: "minutes",
+    roundingIncrement: 30,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 0, 1440, 0, 0, 0, 0);
+}
+
+{
+  const duration = new Temporal.Duration(0, 0, 0, 0, 0, 0, /* seconds = */ 86375);
+  const result = duration.round({
+    relativeTo,
+    smallestUnit: "seconds",
+    roundingIncrement: 30,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 0, 0, 86400, 0, 0, 0);
+}
+
+{
+  const duration = new Temporal.Duration(0, 0, 0, 0, 0, 0, 0, /* milliseconds = */ 86399_650);
+  const result = duration.round({
+    relativeTo,
+    smallestUnit: "milliseconds",
+    roundingIncrement: 500,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 0, 0, 0, 86400_000, 0, 0);
+}
+
+{
+  const duration = new Temporal.Duration(0, 0, 0, 0, 0, 0, 0, 0, /* microseconds = */ 86399_999_650);
+  const result = duration.round({
+    relativeTo,
+    smallestUnit: "microseconds",
+    roundingIncrement: 500,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 0, 0, 0, 0, 86400_000_000, 0);
+}
+
+{
+  const duration = new Temporal.Duration(0, 0, 0, 0, 0, 0, 0, 0, 0, /* nanoseconds = */ 86399_999_999_650);
+  const result = duration.round({
+    relativeTo,
+    smallestUnit: "nanoseconds",
+    roundingIncrement: 500,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 0, 0, 0, 0, 0, 86400_000_000_000);
+}
diff --git a/test/built-ins/Temporal/PlainDateTime/prototype/since/bubble-time-unit.js b/test/built-ins/Temporal/PlainDateTime/prototype/since/bubble-time-unit.js
new file mode 100644
index 0000000000..e480df754b
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDateTime/prototype/since/bubble-time-unit.js
@@ -0,0 +1,83 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.since
+description: Rounds relative to a date, bubbling up time units.
+info: |
+  https://github.com/tc39/proposal-temporal/issues/3121
+  Test case that calls BubbleRelativeDuration with a time unit as largestUnit.
+  This happens when time units round up and overflow a date unit, and
+  largestUnit is not "days" or higher, and the rounding is relative to a
+  PlainDateTime (ZonedDateTime would use Instant semantics in that case.)
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const earlier = new Temporal.PlainDateTime(2025, 6, 14);
+
+{
+  const later = new Temporal.PlainDateTime(2025, 6, 14, 14);
+  const result = later.since(earlier, {
+    largestUnit: "hours",
+    smallestUnit: "hours",
+    roundingIncrement: 12,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 24, 0, 0, 0, 0, 0);
+}
+
+{
+  const later = new Temporal.PlainDateTime(2025, 6, 14, 23, 35);
+  const result = later.since(earlier, {
+    largestUnit: "minutes",
+    smallestUnit: "minutes",
+    roundingIncrement: 30,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 0, 1440, 0, 0, 0, 0);
+}
+
+{
+  const later = new Temporal.PlainDateTime(2025, 6, 14, 23, 59, 35);
+  const result = later.since(earlier, {
+    largestUnit: "seconds",
+    smallestUnit: "seconds",
+    roundingIncrement: 30,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 0, 0, 86400, 0, 0, 0);
+}
+
+{
+  const later = new Temporal.PlainDateTime(2025, 6, 14, 23, 59, 59, 650);
+  const result = later.since(earlier, {
+    largestUnit: "milliseconds",
+    smallestUnit: "milliseconds",
+    roundingIncrement: 500,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 0, 0, 0, 86400_000, 0, 0);
+}
+
+{
+  const later = new Temporal.PlainDateTime(2025, 6, 14, 23, 59, 59, 999, 650);
+  const result = later.since(earlier, {
+    largestUnit: "microseconds",
+    smallestUnit: "microseconds",
+    roundingIncrement: 500,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 0, 0, 0, 0, 86400_000_000, 0);
+}
+
+{
+  const later = new Temporal.PlainDateTime(2025, 6, 14, 23, 59, 59, 999, 999, 650);
+  const result = later.since(earlier, {
+    largestUnit: "nanoseconds",
+    smallestUnit: "nanoseconds",
+    roundingIncrement: 500,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 0, 0, 0, 0, 0, 86400_000_000_000);
+}
diff --git a/test/built-ins/Temporal/PlainDateTime/prototype/until/bubble-time-unit.js b/test/built-ins/Temporal/PlainDateTime/prototype/until/bubble-time-unit.js
new file mode 100644
index 0000000000..7410d5a000
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDateTime/prototype/until/bubble-time-unit.js
@@ -0,0 +1,83 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.until
+description: Rounds relative to a date, bubbling up time units.
+info: |
+  https://github.com/tc39/proposal-temporal/issues/3121
+  Test case that calls BubbleRelativeDuration with a time unit as largestUnit.
+  This happens when time units round up and overflow a date unit, and
+  largestUnit is not "days" or higher, and the rounding is relative to a
+  PlainDateTime (ZonedDateTime would use Instant semantics in that case.)
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const earlier = new Temporal.PlainDateTime(2025, 6, 14);
+
+{
+  const later = new Temporal.PlainDateTime(2025, 6, 14, 14);
+  const result = earlier.until(later, {
+    largestUnit: "hours",
+    smallestUnit: "hours",
+    roundingIncrement: 12,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 24, 0, 0, 0, 0, 0);
+}
+
+{
+  const later = new Temporal.PlainDateTime(2025, 6, 14, 23, 35);
+  const result = earlier.until(later, {
+    largestUnit: "minutes",
+    smallestUnit: "minutes",
+    roundingIncrement: 30,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 0, 1440, 0, 0, 0, 0);
+}
+
+{
+  const later = new Temporal.PlainDateTime(2025, 6, 14, 23, 59, 35);
+  const result = earlier.until(later, {
+    largestUnit: "seconds",
+    smallestUnit: "seconds",
+    roundingIncrement: 30,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 0, 0, 86400, 0, 0, 0);
+}
+
+{
+  const later = new Temporal.PlainDateTime(2025, 6, 14, 23, 59, 59, 650);
+  const result = earlier.until(later, {
+    largestUnit: "milliseconds",
+    smallestUnit: "milliseconds",
+    roundingIncrement: 500,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 0, 0, 0, 86400_000, 0, 0);
+}
+
+{
+  const later = new Temporal.PlainDateTime(2025, 6, 14, 23, 59, 59, 999, 650);
+  const result = earlier.until(later, {
+    largestUnit: "microseconds",
+    smallestUnit: "microseconds",
+    roundingIncrement: 500,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 0, 0, 0, 0, 86400_000_000, 0);
+}
+
+{
+  const later = new Temporal.PlainDateTime(2025, 6, 14, 23, 59, 59, 999, 999, 650);
+  const result = earlier.until(later, {
+    largestUnit: "nanoseconds",
+    smallestUnit: "nanoseconds",
+    roundingIncrement: 500,
+    roundingMode: "ceil"
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 0, 0, 0, 0, 0, 86400_000_000_000);
+}