diff --git a/packages/core/clockworker.js b/packages/core/clockworker.js
index db9ef123..e16da478 100644
--- a/packages/core/clockworker.js
+++ b/packages/core/clockworker.js
@@ -1,5 +1,8 @@
 // eslint-disable-next-line no-undef
 importScripts('./neozyklus.js');
+// TODO: swap below line with above one when firefox supports esm imports in service workers
+// see https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorker?retiredLocale=de#browser_compatibility
+// import createClock from './zyklus.mjs';
 
 function getTime() {
   const precision = 10 ** 4;
diff --git a/packages/core/zyklus.mjs b/packages/core/zyklus.mjs
index 3d25b054..d3c48667 100644
--- a/packages/core/zyklus.mjs
+++ b/packages/core/zyklus.mjs
@@ -23,7 +23,7 @@ function createClock(
     // callback as long as we're inside the lookahead
     while (phase < lookahead) {
       phase = Math.round(phase * precision) / precision;
-      phase >= t && callback(phase, duration, tick);
+      phase >= t && callback(phase, duration, tick, t);
       phase < t && console.log('TOO LATE', phase); // what if latency is added from outside?
       phase += duration; // increment phase by duration
       tick++;