diff --git a/widget/cocoa/nsCocoaUtils.h b/widget/cocoa/nsCocoaUtils.h
index c1cc7fbc6949..a9f267b62d7c 100644
--- a/widget/cocoa/nsCocoaUtils.h
+++ b/widget/cocoa/nsCocoaUtils.h
@@ -34,6 +34,7 @@ enum {
 class nsIWidget;
 
 namespace mozilla {
+class TimeStamp;
 namespace gfx {
 class SourceSurface;
 } // namespace gfx
@@ -377,6 +378,12 @@ public:
            const nsTArray<mozilla::FontRange>& aFontRanges,
            const bool aIsVertical,
            const CGFloat aBackingScaleFactor);
+
+  /**
+   * Compute TimeStamp from an event's timestamp.
+   * If aEventTime is 0, this returns current timestamp.
+   */
+  static mozilla::TimeStamp GetEventTimeStamp(NSTimeInterval aEventTime);
 };
 
 #endif // nsCocoaUtils_h_
diff --git a/widget/cocoa/nsCocoaUtils.mm b/widget/cocoa/nsCocoaUtils.mm
index 3138245aa785..4ed439592930 100644
--- a/widget/cocoa/nsCocoaUtils.mm
+++ b/widget/cocoa/nsCocoaUtils.mm
@@ -1020,3 +1020,22 @@ nsCocoaUtils::GetNSMutableAttributedString(
 
   NS_OBJC_END_TRY_ABORT_BLOCK_NIL
 }
+
+TimeStamp
+nsCocoaUtils::GetEventTimeStamp(NSTimeInterval aEventTime)
+{
+  if (!aEventTime) {
+    // If the event is generated by a 3rd party application, its timestamp
+    // may be 0.  In this case, just return current timestamp.
+    // XXX Should we cache last event time?
+    return TimeStamp::Now();
+  }
+  // The internal value of the macOS implementation of TimeStamp is based on
+  // mach_absolute_time(), which measures "ticks" since boot.
+  // Event timestamps are NSTimeIntervals (seconds) since boot. So the two time
+  // representations already have the same base; we only need to convert
+  // seconds into ticks.
+  int64_t tick =
+    BaseTimeDurationPlatformUtils::TicksFromMilliseconds(aEventTime * 1000.0);
+  return TimeStamp::FromSystemTime(tick);
+}