diff --git a/services/experimentAnalysis.ts b/services/experimentAnalysis.ts
index ed172ba..3ad6918 100644
--- a/services/experimentAnalysis.ts
+++ b/services/experimentAnalysis.ts
@@ -42,10 +42,10 @@ interface ComputedWeeklyMetrics extends WeeklyMetrics {
   roas: number;
 }
 
-function getISOWeekStart(year: number, week: number): number {
-  const jan4 = new Date(year, 0, 4);
-  const day = jan4.getDay() || 7;
-  const startYear = new Date(year, 0, 4 - day + 1);
+function getWeekStartSunday(year: number, week: number): number {
+  const jan1 = new Date(year, 0, 1);
+  const day = jan1.getDay(); // 0 = Sunday, 1 = Monday...
+  const startYear = new Date(year, 0, 1 - day); // Sunday of the week containing Jan 1
   return startYear.getTime() + (week - 1) * 7 * 86400000;
 }
 
@@ -70,7 +70,7 @@ function aggregateWeeklyMetrics(
     if (!weeklyMap.has(key)) {
       weeklyMap.set(key, {
         week: key,
-        timestamp: getISOWeekStart(year, weekNum),
+        timestamp: getWeekStartSunday(year, weekNum),
         units: 0,
         revenue: 0,
         sessions: 0,