-    const requestedStart = dayjs("2026-07-15T10:00:00Z");
-    const requestedEnd = dayjs("2026-07-15T11:00:00Z");
+    const requestedStart = new Date("2026-07-15T10:00:00Z");
+    const requestedEnd = new Date("2026-07-15T11:00:00Z");

     const setupPrismaMock = (existingBookings: any[]) => {
       vi.mocked(prisma.booking.findMany).mockImplementation(async (args) => {
         const where = args?.where as any;
         const startTimeCondition = where?.startTime;
         const endTimeCondition = where?.endTime;
         
         return existingBookings.filter((booking) => {
-          // Emulate Prisma's lt/gt logic
-          const startsBeforeEnd = startTimeCondition?.lt
-            ? dayjs(booking.startTime).isBefore(dayjs(startTimeCondition.lt))
-            : true;
-          const endsAfterStart = endTimeCondition?.gt
-            ? dayjs(booking.endTime).isAfter(dayjs(endTimeCondition.gt))
-            : true;
+          // Emulate Prisma's lt/gt logic
+          const startsBeforeEnd = startTimeCondition?.lt
+            ? new Date(booking.startTime).getTime() < new Date(startTimeCondition.lt).getTime()
+            : true;
+          const endsAfterStart = endTimeCondition?.gt
+            ? new Date(booking.endTime).getTime() > new Date(endTimeCondition.gt).getTime()
+            : true;
           return startsBeforeEnd && endsAfterStart;
         });
       });
     };

     // ... (unchanged array blocks)

     ])("$scenario", async ({ existingStart, existingEnd, expectedCount }) => {
       const mockBooking = createMockBookingResult({
-        startTime: dayjs(existingStart).toDate(),
-        endTime: dayjs(existingEnd).toDate(),
+        startTime: new Date(existingStart),
+        endTime: new Date(existingEnd),
       });
       setupPrismaMock([mockBooking]);

       const busyTimesService = getBusyTimesService();
       const busyTimes = await busyTimesService.getBusyTimesForLimitChecks({
         userIds: [1],
         eventTypeId: 1,
-        startDate: requestedStart.format(),
-        endDate: requestedEnd.format(),
+        startDate: requestedStart.toISOString(),
+        endDate: requestedEnd.toISOString(),
         // PER_YEAR is explicitly excluded from date range expansion in getStartEndDateforLimitCheck
         // This allows us to test the exact requested interval [10:00 - 11:00]
         bookingLimits: { PER_YEAR: 5 },
       });

-        const where = args?.where as any;
+        const where = args?.where as { startTime?: { lt?: Date }; endTime?: { gt?: Date } } | undefined;