Add time units for duration

files/en-us/web/api/performance/measure/index.md

@@ -49,7 +49,7 @@ To only provide an `endMark`, you need to provide an empty `measureOptions` obje
         If this is a string naming a {{domxref("PerformanceMark")}}, then it is defined in the same way as `startMark`.
 
     - `duration` {{optional_inline}}
-      - : Duration between the start and end mark times. If omitted, this defaults to {{domxref("performance.now()")}}; the time that has elapsed since the context was created. If provided, you must also specify either `start` or `end` but not both.
+      - : Duration (in milliseconds) between the start and end mark times. If omitted, this defaults to {{domxref("performance.now()")}}; the time that has elapsed since the context was created. If provided, you must also specify either `start` or `end` but not both.
     - `end` {{optional_inline}}
 
       - : Timestamp ({{domxref("DOMHighResTimeStamp")}}) to be used as the end time, or string that names a {{domxref("PerformanceMark")}} to use for the end time.