diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index 1eb1d0ff4d72..c8608fde93b4 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -125,6 +125,7 @@ export {
   getActiveSpan,
   addChildSpanToSpan,
   spanTimeInputToSeconds,
+  otelTimeInputToSeconds,
   updateSpanName,
   INTERNAL_setSegmentNameSourceIfSegment,
 } from './utils/spanUtils';
diff --git a/packages/core/src/tracing/idleSpan.ts b/packages/core/src/tracing/idleSpan.ts
index 32e23ee357ba..e34d522813db 100644
--- a/packages/core/src/tracing/idleSpan.ts
+++ b/packages/core/src/tracing/idleSpan.ts
@@ -322,7 +322,8 @@ export function startIdleSpan(startSpanOptions: StartSpanOptions, options: Parti
       // We cancel all pending spans with status "cancelled" to indicate the idle span was finished early
       if (childSpan.isRecording()) {
         childSpan.setStatus({ code: SPAN_STATUS_ERROR, message: 'cancelled' });
-        childSpan.end(endTimestamp);
+        // Provider-created descendants use OTel's millisecond-based numeric timestamps.
+        childSpan.end([Math.floor(endTimestamp), (endTimestamp % 1) * 1e9]);
         DEBUG_BUILD &&
           debug.log('[Tracing] Cancelling span since span ended early', JSON.stringify(childSpan, undefined, 2));
       }
diff --git a/packages/core/src/tracing/sentrySpan.ts b/packages/core/src/tracing/sentrySpan.ts
index 42d91f6057d6..d71069f4fcb5 100644
--- a/packages/core/src/tracing/sentrySpan.ts
+++ b/packages/core/src/tracing/sentrySpan.ts
@@ -38,6 +38,7 @@ import {
   INTERNAL_setSegmentNameSourceIfSegment,
   spanIsSegment,
   spanTimeInputToSeconds,
+  otelTimeInputToSeconds,
   spanToStaticSpanJSON,
   spanToTransactionTraceContext,
   TRACE_FLAG_NONE,
@@ -253,7 +254,12 @@ export class SentrySpan implements Span {
       return;
     }
 
-    this._endTime = endTimestamp == null ? this._getEndTimeFromDuration() : spanTimeInputToSeconds(endTimestamp);
+    this._endTime =
+      endTimestamp == null
+        ? this._getEndTimeFromDuration()
+        : spanIsTracerProviderSpan(this)
+          ? otelTimeInputToSeconds(endTimestamp)
+          : spanTimeInputToSeconds(endTimestamp);
     logSpanEnd(this);
 
     this._onSpanEnded();
diff --git a/packages/core/src/utils/spanUtils.ts b/packages/core/src/utils/spanUtils.ts
index e1615b9bf5c4..4a9af57eed14 100644
--- a/packages/core/src/utils/spanUtils.ts
+++ b/packages/core/src/utils/spanUtils.ts
@@ -30,7 +30,7 @@ import type {
 import type { SpanStatus } from '../types/spanStatus';
 import { addNonEnumerableProperty } from '../utils/object';
 import { generateSpanId } from '../utils/propagationContext';
-import { timestampInSeconds } from '../utils/time';
+import { browserPerformanceTimeOrigin, timestampInSeconds } from '../utils/time';
 import { generateSentryTraceHeader, generateTraceparentHeader } from '../utils/tracing';
 import { _getSpanForScope } from './spanOnScope';
 
@@ -156,6 +156,22 @@ export function spanTimeInputToSeconds(input: SpanTimeInput | undefined): number
   return timestampInSeconds();
 }
 
+/** Convert OpenTelemetry TimeInput, whose numeric values are milliseconds, to Unix seconds. */
+export function otelTimeInputToSeconds(input: SpanTimeInput | undefined): number {
+  if (typeof input === 'number') {
+    const origin = browserPerformanceTimeOrigin(input);
+    // Match OTel's distinction between epoch milliseconds and performance.now(),
+    // including epoch values just before the process's time origin.
+    return (origin !== undefined && input < origin / 2 ? origin + input : input) / 1000;
+  }
+
+  if (input instanceof Date) {
+    return input.getTime() / 1000;
+  }
+
+  return spanTimeInputToSeconds(input);
+}
+
 /**
  * Converts a timestamp to second, if it was in milliseconds, or keeps it as second.
  */
diff --git a/packages/core/test/lib/tracing/idleSpan.test.ts b/packages/core/test/lib/tracing/idleSpan.test.ts
index a129c35e6a13..4c761b005eae 100644
--- a/packages/core/test/lib/tracing/idleSpan.test.ts
+++ b/packages/core/test/lib/tracing/idleSpan.test.ts
@@ -17,6 +17,7 @@ import {
   startSpanManual,
 } from '../../../src';
 import { startIdleSpan, TRACING_DEFAULTS } from '../../../src/tracing/idleSpan';
+import { markSpanAsTracerProviderSpan } from '../../../src/tracing/utils';
 import type { Event } from '../../../src/types/event';
 import type { Span } from '../../../src/types/span';
 import { getDefaultTestClientOptions, TestClient } from '../../mocks/client';
@@ -58,6 +59,17 @@ describe('startIdleSpan', () => {
     expect(getActiveSpan()).toBe(undefined);
   });
 
+  it('cancels a provider-created child using the idle span end timestamp', () => {
+    const endTimestamp = Date.now() / 1000 + 1;
+    const idleSpan = startIdleSpan({ name: 'idle span' });
+    const child = startInactiveSpan({ name: 'OpenTelemetry child' });
+    markSpanAsTracerProviderSpan(child);
+
+    idleSpan.end(endTimestamp);
+
+    expect(spanToJSON(child).end_timestamp).toBe(endTimestamp);
+  });
+
   it('returns non recording span if tracing is disabled', () => {
     const options = getDefaultTestClientOptions({ dsn });
     const client = new TestClient(options);
diff --git a/packages/core/test/lib/tracing/sentrySpan.test.ts b/packages/core/test/lib/tracing/sentrySpan.test.ts
index ad49179d7ce2..c26895e4d0ce 100644
--- a/packages/core/test/lib/tracing/sentrySpan.test.ts
+++ b/packages/core/test/lib/tracing/sentrySpan.test.ts
@@ -169,8 +169,8 @@ describe('SentrySpan', () => {
       });
       markSpanAsTracerProviderSpan(span);
 
-      span.end(2);
-      span.end(3);
+      span.end([2, 0]);
+      span.end([3, 0]);
 
       span.setAttribute('key', 'after');
       expect(spanToStaticSpanJSON(span).data?.['key']).toBe('before');
diff --git a/packages/opentelemetry/src/tracer.ts b/packages/opentelemetry/src/tracer.ts
index 3b93f4fe8e51..34e0b932c13c 100644
--- a/packages/opentelemetry/src/tracer.ts
+++ b/packages/opentelemetry/src/tracer.ts
@@ -10,6 +10,7 @@ import {
   getCurrentScope,
   getIsolationScope,
   markSpanAsTracerProviderSpan,
+  otelTimeInputToSeconds,
   SentryNonRecordingSpan,
   setCapturedScopesOnSpan,
   spanIsIgnored,
@@ -136,7 +137,7 @@ export class SentryTracer implements Tracer {
       name,
       attributes: (options.attributes as SpanAttributes) || {},
       links: convertSpanLinkType(options.links),
-      startTime: options.startTime,
+      startTime: options.startTime === undefined ? undefined : otelTimeInputToSeconds(options.startTime),
     };
 
     // Convert otel kind to our sentry.kind attribtue
diff --git a/packages/opentelemetry/test/tracerProvider.test.ts b/packages/opentelemetry/test/tracerProvider.test.ts
index 878c878a1af1..17f14213d012 100644
--- a/packages/opentelemetry/test/tracerProvider.test.ts
+++ b/packages/opentelemetry/test/tracerProvider.test.ts
@@ -45,6 +45,43 @@ describe('SentryTracerProvider', () => {
     });
   });
 
+  it('anchors performance timestamps to the Unix epoch and preserves millisecond durations', () => {
+    const before = Date.now() / 1000;
+    const start = performance.now();
+    const span = trace.getTracer('test').startSpan('NextNodeServer.clientComponentLoading', { startTime: start });
+
+    span.end(start + 25);
+
+    const json = spanToJSON(span as Span);
+    expect(json.start_timestamp).toBeGreaterThanOrEqual(before - 1);
+    expect(json.start_timestamp).toBeLessThanOrEqual(Date.now() / 1000 + 1);
+    expect(json.end_timestamp! - json.start_timestamp).toBeCloseTo(0.025, 5);
+  });
+
+  it('accepts zero as a performance timestamp', () => {
+    const span = trace.getTracer('test').startSpan('performance origin', { startTime: 0 });
+
+    span.end(25);
+
+    const json = spanToJSON(span as Span);
+    expect(json.start_timestamp).toBeCloseTo(performance.timeOrigin / 1000, 2);
+    expect(json.end_timestamp! - json.start_timestamp).toBeCloseTo(0.025, 5);
+  });
+
+  it.each([
+    { startTime: 1700000000000, endTime: 1700000000025 },
+    { startTime: new Date(1700000000000), endTime: new Date(1700000000025) },
+    { startTime: [1700000000, 0] as [number, number], endTime: [1700000000, 25000000] as [number, number] },
+  ])('preserves absolute OpenTelemetry timestamps ($startTime)', ({ startTime, endTime }) => {
+    const span = trace.getTracer('test').startSpan('absolute timestamp', { startTime });
+
+    span.end(endTime);
+
+    const json = spanToJSON(span as Span);
+    expect(json.start_timestamp).toBe(1700000000);
+    expect(json.end_timestamp).toBe(1700000000.025);
+  });
+
   it('parents inactive spans to the active OpenTelemetry span', () => {
     trace.getTracer('test').startActiveSpan('parent', parent => {
       const child = trace.getTracer('test').startSpan('child');