--- a/widget/cocoa/nsCocoaUtils.h
+++ b/widget/cocoa/nsCocoaUtils.h
@@ -29,16 +29,17 @@
 enum {
   NSEventPhaseMayBegin    = 0x1 << 5
 };
 #endif
 
 class nsIWidget;
 
 namespace mozilla {
+class TimeStamp;
 namespace gfx {
 class SourceSurface;
 } // namespace gfx
 } // namespace mozilla
 
 // Used to retain a Cocoa object for the remainder of a method's execution.
 class nsAutoRetainCocoaObject {
 public:
@@ -372,11 +373,17 @@ public:
   /**
    * Convert string with font attribute to NSMutableAttributedString
    */
   static NSMutableAttributedString* GetNSMutableAttributedString(
            const nsAString& aText,
            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_

--- a/widget/cocoa/nsCocoaUtils.mm
+++ b/widget/cocoa/nsCocoaUtils.mm
@@ -1015,8 +1015,27 @@ nsCocoaUtils::GetNSMutableAttributedStri
                     value:[NSNumber numberWithInt: 1]
                     range:NSMakeRange(0, [attrStr length])];
   }
 
   return attrStr;
 
   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);
+}