From 4de9fb5b89dc4346755a3996e14f4ada5b4fefcd Mon Sep 17 00:00:00 2001
From: Christian Vidal Wolf <christianvidalwolf@gmail.com>
Date: Wed, 25 Feb 2026 08:39:13 +0100
Subject: [PATCH] fix(experiments): include overlapping weeks in date range
 logic and use exact ISO weeks

---
 services/experimentAnalysis.ts | 34 ++++++++++++++++++++++++++--------
 1 file changed, 26 insertions(+), 8 deletions(-)

diff --git a/services/experimentAnalysis.ts b/services/experimentAnalysis.ts
index e80bd55..651d538 100644
--- a/services/experimentAnalysis.ts
+++ b/services/experimentAnalysis.ts
@@ -42,6 +42,13 @@ 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);
+  return startYear.getTime() + (week - 1) * 7 * 86400000;
+}
+
 function aggregateWeeklyMetrics(
   asinSet: Set<string>,
   salesData: CombinedKPIs[],
@@ -61,10 +68,9 @@ function aggregateWeeklyMetrics(
     const key = `${year}-W${String(weekNum).padStart(2, '0')}`;
 
     if (!weeklyMap.has(key)) {
-      const d = new Date(year, 0, 1 + (weekNum - 1) * 7);
       weeklyMap.set(key, {
         week: key,
-        timestamp: d.getTime(),
+        timestamp: getISOWeekStart(year, weekNum),
         units: 0,
         revenue: 0,
         sessions: 0,
@@ -125,10 +131,20 @@ function splitPeriods(
   const after: ComputedWeeklyMetrics[] = [];
 
   for (const w of data) {
-    if (w.timestamp >= beforeStartTs && w.timestamp < beforeEndTs) {
-      before.push(w);
-    } else if (w.timestamp >= startTs && w.timestamp <= endTs) {
+    const weekStartTs = w.timestamp;
+    const weekEndTs = w.timestamp + 6 * 86400000 + 86399999; // End of the 7th day
+
+    // A week overlaps the treatment period if it starts before the period ends, 
+    // AND ends after the period starts.
+    const overlapsAfter = weekEndTs >= startTs && weekStartTs <= endTs;
+    const overlapsBefore = weekEndTs >= beforeStartTs && weekStartTs < beforeEndTs;
+
+    // We prioritize assigning to 'after' so that we capture all units happening 
+    // closely around the experiment dates for accurate reporting.
+    if (overlapsAfter) {
       after.push(w);
+    } else if (overlapsBefore) {
+      before.push(w);
     }
   }
 
@@ -322,12 +338,13 @@ export function buildCounterfactualSeries(
   if (!hasControlGroup) {
     // Without control group, counterfactual = flat line at pre-treatment average
     const startTs = startDate.getTime();
+    const endTs = endDate.getTime();
     const beforeStartTs = beforeStart.getTime();
     const beforeEndTs = beforeEnd.getTime();
 
     // To calculate counterfactual, we need the "before" average.
     // Treatment before period:
-    const beforeData = treatmentWeekly.filter(w => w.timestamp >= beforeStartTs && w.timestamp < beforeEndTs);
+    const { before: beforeData } = splitPeriods(treatmentWeekly, startTs, endTs, beforeStartTs, beforeEndTs); // Use splitPeriods to match exactly
     const preAvg = avgMetric(beforeData, metric);
 
     return treatmentWeekly.map(w => ({
@@ -342,11 +359,12 @@ export function buildCounterfactualSeries(
   const controlWeekly = aggregateWeeklyMetrics(controlAsinSet, salesData, experiment.marketplace);
 
   const startTs = startDate.getTime();
+  const endTs = endDate.getTime();
   const beforeStartTs = beforeStart.getTime();
   const beforeEndTs = beforeEnd.getTime();
 
-  const tBeforeData = treatmentWeekly.filter(w => w.timestamp >= beforeStartTs && w.timestamp < beforeEndTs);
-  const cBeforeData = controlWeekly.filter(w => w.timestamp >= beforeStartTs && w.timestamp < beforeEndTs);
+  const { before: tBeforeData } = splitPeriods(treatmentWeekly, startTs, endTs, beforeStartTs, beforeEndTs);
+  const { before: cBeforeData } = splitPeriods(controlWeekly, startTs, endTs, beforeStartTs, beforeEndTs);
 
   const tPreAvg = avgMetric(tBeforeData, metric);
   const cPreAvg = avgMetric(cBeforeData, metric);