angularfire2 collection querying with endAt wont give result

I am trying to querying date range at firebase by using startAt() and endAt() methods.(Angular6)
For date picking i use angular material MatDatepicker module.

q_startDate; q_endDate;
getStartDate(event: MatDatepickerInputEvent<Date>) {
this.q_startDate = event.value.toISOString();}


I defined q_startDate and q_endDate veriables and pushed data with getStartDate and getEndDate methods.

const myCollection = this.afs.collection<objInterface>('collection_name', ref=> {
let query: firestore.CollectionReference | firestore.Query = ref;
query = query.orderBy('date_field', 'desc');
query = query.startAt(this.q_startDate);
query = query.endAt(this.q_endDate);
return query;
});


In this query when i comment query = query.endAt(this.q_endDate); it works.
How can i fix this issue, i want to include query also q_endDate?

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