fix most type errors

still have a few things outstanding

helpers/date.ts

@@ -3,7 +3,13 @@ import moment from 'moment'
 const FRONTEND_FORMAT = 'DD.MM.YYYY'
 const BACKEND_FORMAT = 'YYYY-MM-DD'
 
-export function getDays({ startDate, endDate }) {
+export function getDays({
+  startDate,
+  endDate,
+}: {
+  startDate: moment.MomentInput
+  endDate: moment.MomentInput
+}) {
   let currentDay = moment(startDate)
   const days = [dateFormatBackend(currentDay)]
 
@@ -20,31 +26,30 @@ export function getDays({ startDate, endDate }) {
   return days
 }
 
-function dateFormat(date, format) {
+function dateFormat(date: moment.MomentInput, format: string) {
   if (!date) {
     return null
   }
   return moment(date).format(format)
 }
 
-export function dateFormatBackend(date) {
+export function dateFormatBackend(date: moment.MomentInput) {
   return dateFormat(date, BACKEND_FORMAT)
 }
 
-export function dateFormatFrontend(date) {
+export function dateFormatFrontend(date: moment.MomentInput) {
   return dateFormat(date, FRONTEND_FORMAT)
 }
 
-function dateParse(string, format) {
-  const date = moment(string, format)
+function dateParse(input: string, format: string) {
+  const date = moment(input, format)
   if (date.isValid()) {
-    // @ts-expect-error ts-migrate(2339) FIXME: Property 'getDate' does not exist on type 'Moment'... Remove this comment to see the full error message
-    return date.getDate()
+    return date.toDate()
   }
 
   return null
 }
 
-export function dateParseFrontend(string) {
-  return dateParse(string, FRONTEND_FORMAT)
+export function dateParseFrontend(input: string) {
+  return dateParse(input, FRONTEND_FORMAT)
 }