--- a/toolkit/components/osfile/modules/osfile_async_front.jsm
+++ b/toolkit/components/osfile/modules/osfile_async_front.jsm
@@ -161,19 +161,19 @@ let Scheduler = {
         }
         // Bug 874425 demonstrates that two successive calls to Date.now()
         // can actually produce an interval with negative duration.
         // We assume that this is due to an operation that is so short
         // that Date.now() is not monotonic, so we round this up to 0.
         let durationMs = Math.max(0, data.durationMs);
         // Accumulate (or initialize) outExecutionDuration
         if (typeof options.outExecutionDuration == "number") {
-          options.outExecutionDuration += data.durationMs;
+          options.outExecutionDuration += durationMs;
         } else {
-          options.outExecutionDuration = data.durationMs;
+          options.outExecutionDuration = durationMs;
         }
         return data.ok;
       },
       function onError(error) {
         // Decode any serialized error
         if (error instanceof PromiseWorker.WorkerError) {
           throw OS.File.Error.fromMsg(error.data);
         } else {