diff --git a/packages/core/src/utils/time.ts b/packages/core/src/utils/time.ts
index ff858a15b0ac..bfed9386f8bb 100644
--- a/packages/core/src/utils/time.ts
+++ b/packages/core/src/utils/time.ts
@@ -74,22 +74,23 @@ export function timestampInSeconds(): number {
 /**
  * Cached result of getBrowserTimeOrigin.
  */
-let cachedTimeOrigin: [number | undefined, string] | undefined;
+let cachedTimeOrigin: number | null | undefined = null;
 
 /**
  * Gets the time origin and the mode used to determine it.
+ * TODO: move to `@sentry/browser-utils` package.
  */
-function getBrowserTimeOrigin(): [number | undefined, string] {
+function getBrowserTimeOrigin(): number | undefined {
   // Unfortunately browsers may report an inaccurate time origin data, through either performance.timeOrigin or
   // performance.timing.navigationStart, which results in poor results in performance data. We only treat time origin
   // data as reliable if they are within a reasonable threshold of the current time.
-
   const { performance } = GLOBAL_OBJ as typeof GLOBAL_OBJ & Window;
   if (!performance?.now) {
-    return [undefined, 'none'];
+    return undefined;
   }
 
-  const threshold = 3600 * 1000;
+  // TOOD: We should probably set a much tighter threshold here as skew can already happen within just a few minutes.
+  const threshold = 3_600_000; // 1 hour in milliseconds
   const performanceNow = performance.now();
   const dateNow = Date.now();
 
@@ -99,6 +100,10 @@ function getBrowserTimeOrigin(): [number | undefined, string] {
     : threshold;
   const timeOriginIsReliable = timeOriginDelta < threshold;
 
+  // TODO: Remove all code related to `performance.timing.navigationStart` once we drop support for Safari 14.
+  // `performance.timeSince` is available in Safari 15.
+  // see: https://caniuse.com/mdn-api_performance_timeorigin
+
   // While performance.timing.navigationStart is deprecated in favor of performance.timeOrigin, performance.timeOrigin
   // is not as widely supported. Namely, performance.timeOrigin is undefined in Safari as of writing.
   // Also as of writing, performance.timing is not available in Web Workers in mainstream browsers, so it is not always
@@ -111,17 +116,18 @@ function getBrowserTimeOrigin(): [number | undefined, string] {
   const navigationStartDelta = hasNavigationStart ? Math.abs(navigationStart + performanceNow - dateNow) : threshold;
   const navigationStartIsReliable = navigationStartDelta < threshold;
 
-  if (timeOriginIsReliable || navigationStartIsReliable) {
-    // Use the more reliable time origin
-    if (timeOriginDelta <= navigationStartDelta) {
-      return [performance.timeOrigin, 'timeOrigin'];
-    } else {
-      return [navigationStart, 'navigationStart'];
-    }
+  // TODO: Since timeOrigin explicitly replaces navigationStart, we should probably remove the navigationStartIsReliable check.
+  if (timeOriginIsReliable && timeOriginDelta <= navigationStartDelta) {
+    return performance.timeOrigin;
+  }
+
+  if (navigationStartIsReliable) {
+    return navigationStart;
   }
 
+  // TODO: We should probably fall back to Date.now() - performance.now(), since this is still more accurate than just Date.now() (?)
   // Either both timeOrigin and navigationStart are skewed or neither is available, fallback to Date.
-  return [dateNow, 'dateNow'];
+  return dateNow;
 }
 
 /**
@@ -129,9 +135,9 @@ function getBrowserTimeOrigin(): [number | undefined, string] {
  * performance API is available.
  */
 export function browserPerformanceTimeOrigin(): number | undefined {
-  if (!cachedTimeOrigin) {
+  if (cachedTimeOrigin === null) {
     cachedTimeOrigin = getBrowserTimeOrigin();
   }
 
-  return cachedTimeOrigin[0];
+  return cachedTimeOrigin;
 }
diff --git a/packages/core/test/lib/utils/time.test.ts b/packages/core/test/lib/utils/time.test.ts
new file mode 100644
index 000000000000..e40c607cb409
--- /dev/null
+++ b/packages/core/test/lib/utils/time.test.ts
@@ -0,0 +1,142 @@
+import { describe, expect, it, vi } from 'vitest';
+
+async function getFreshPerformanceTimeOrigin() {
+  // Adding the query param with the date, forces a fresh import each time this is called
+  // otherwise, the dynamic import would be cached and thus fall back to the cached value.
+  const timeModule = await import(`../../../src/utils/time?update=${Date.now()}`);
+  return timeModule.browserPerformanceTimeOrigin();
+}
+
+const RELIABLE_THRESHOLD_MS = 3_600_000;
+
+describe('browserPerformanceTimeOrigin', () => {
+  it('returns `performance.timeOrigin` if it is available and reliable', async () => {
+    const timeOrigin = await getFreshPerformanceTimeOrigin();
+    expect(timeOrigin).toBeDefined();
+    expect(timeOrigin).toBeGreaterThan(0);
+    expect(timeOrigin).toBeLessThan(Date.now());
+    expect(timeOrigin).toBe(performance.timeOrigin);
+  });
+
+  it('returns `undefined` if `performance.now` is not available', async () => {
+    vi.stubGlobal('performance', undefined);
+
+    const timeOrigin = await getFreshPerformanceTimeOrigin();
+    expect(timeOrigin).toBeUndefined();
+
+    vi.unstubAllGlobals();
+  });
+
+  it('returns `Date.now()` if `performance.timeOrigin` is not reliable', async () => {
+    const currentTimeMs = 1767778040866;
+
+    const unreliableTime = currentTimeMs - RELIABLE_THRESHOLD_MS - 2_000;
+
+    const timeSincePageloadMs = 1_234.789;
+
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date(currentTimeMs));
+
+    vi.stubGlobal('performance', {
+      timeOrigin: unreliableTime,
+      timing: {
+        navigationStart: unreliableTime,
+      },
+      now: () => timeSincePageloadMs,
+    });
+
+    const timeOrigin = await getFreshPerformanceTimeOrigin();
+    expect(timeOrigin).toBe(1767778040866);
+
+    vi.useRealTimers();
+    vi.unstubAllGlobals();
+  });
+
+  it('returns `performance.timing.navigationStart` if `performance.timeOrigin` is not available', async () => {
+    const currentTimeMs = 1767778040870;
+
+    const navigationStartMs = currentTimeMs - 2_000;
+
+    const timeSincePageloadMs = 1_234.789;
+
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date(currentTimeMs));
+
+    vi.stubGlobal('performance', {
+      timeOrigin: undefined,
+      timing: {
+        navigationStart: navigationStartMs,
+      },
+      now: () => timeSincePageloadMs,
+    });
+
+    const timeOrigin = await getFreshPerformanceTimeOrigin();
+    expect(timeOrigin).toBe(navigationStartMs);
+
+    vi.useRealTimers();
+    vi.unstubAllGlobals();
+  });
+
+  it('returns `performance.timing.navigationStart` if `performance.timeOrigin` is less reliable', async () => {
+    const currentTimeMs = 1767778040874;
+
+    const navigationStartMs = currentTimeMs - 2_000;
+
+    const timeSincePageloadMs = 1_234.789;
+
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date(currentTimeMs));
+
+    vi.stubGlobal('performance', {
+      timeOrigin: navigationStartMs - 1,
+      timing: {
+        navigationStart: navigationStartMs,
+      },
+      now: () => timeSincePageloadMs,
+    });
+
+    const timeOrigin = await getFreshPerformanceTimeOrigin();
+    expect(timeOrigin).toBe(navigationStartMs);
+
+    vi.useRealTimers();
+    vi.unstubAllGlobals();
+  });
+
+  describe('caching', () => {
+    it('caches `undefined` result', async () => {
+      vi.stubGlobal('performance', undefined);
+
+      const timeModule = await import(`../../../src/utils/time?update=${Date.now()}`);
+
+      const result1 = timeModule.browserPerformanceTimeOrigin();
+
+      expect(result1).toBeUndefined();
+
+      vi.stubGlobal('performance', {
+        timeOrigin: 1000,
+        now: () => 100,
+      });
+
+      const result2 = timeModule.browserPerformanceTimeOrigin();
+      expect(result2).toBeUndefined(); // Should still be undefined due to caching
+
+      vi.unstubAllGlobals();
+    });
+
+    it('caches `number` result', async () => {
+      const timeModule = await import(`../../../src/utils/time?update=${Date.now()}`);
+      const result = timeModule.browserPerformanceTimeOrigin();
+      const timeOrigin = performance.timeOrigin;
+      expect(result).toBe(timeOrigin);
+
+      vi.stubGlobal('performance', {
+        now: undefined,
+      });
+
+      const result2 = timeModule.browserPerformanceTimeOrigin();
+      expect(result2).toBe(timeOrigin);
+
+      vi.unstubAllGlobals();
+    });
+  });
+});