use strings for startDate endDate

all the way to the FE

db/booking.ts

@@ -1,6 +1,5 @@
 import * as mongoose from 'mongoose'
 import { v4 as uuidv4 } from 'uuid'
-import { startOfDay } from 'date-fns'
 import { dateFormatBackend, getDays } from '../helpers/date'
 import { Bill } from './bill'
 import { Booker } from './booker'
@@ -10,8 +9,8 @@ export interface Booking {
   uuid: string
   booker: Booker
   bill: Bill
-  startDate: Date
-  endDate: Date
+  startDate: string
+  endDate: string
   status: BOOKING_STATUS
   purpose?: string
   org?: string
@@ -94,7 +93,8 @@ BookingSchema.static('findBookedDays', async function (): Promise<string[]> {
     .find(
       {
         status: { $in: [BOOKING_STATUS.REQUESTED, BOOKING_STATUS.CONFIRMED] },
-        endDate: { $gt: startOfDay(new Date()) },
+        // dateFormatBackend uses YYYY-MM-DD, which is startOfDay anyway
+        endDate: { $gt: dateFormatBackend(new Date()) },
       },
       'days'
     )