diff --git a/frontend/src/pages/WeeklyActual.tsx b/frontend/src/pages/WeeklyActual.tsx
index 89b3d216..5cc250b2 100644
--- a/frontend/src/pages/WeeklyActual.tsx
+++ b/frontend/src/pages/WeeklyActual.tsx
@@ -31,19 +31,22 @@ const MONTH_NAMES = ['January','February','March','April','May','June','July','A
 
 // ── Week navigation ────────────────────────────────────────────────────────
 
+function localDateStr(d: Date): string {
+  return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, '0')}-${String(d.getDate()).padStart(2, '0')}`
+}
+
 function defaultWeekEnding(): string {
   const d = new Date()
   const day = d.getDay()
-  // Go back to the most recent Saturday (day=6→today, else go back)
   const daysBack = day === 6 ? 0 : day + 1
   d.setDate(d.getDate() - daysBack)
-  return d.toISOString().split('T')[0]
+  return localDateStr(d)
 }
 
 function shiftWeek(dateStr: string, weeks: number): string {
   const d = new Date(dateStr + 'T00:00:00')
   d.setDate(d.getDate() + weeks * 7)
-  return d.toISOString().split('T')[0]
+  return localDateStr(d)
 }
 
 // ── Chart colours ──────────────────────────────────────────────────────────