wrong date on conversion toISOString()

I am confused why on converting “8/20/2021” to ISO(cosmosDB format) I am getting “2021-08-19T18:30:00.000Z”?

const event = new Date('8/20/2021');
console.log(event.toISOString()); //"2021-08-19T18:30:00.000Z"

all the date are saved as string in my cosmosdb and hence I was not able to filter based on given date range and as result we ended up converting string to Date but now when I am trying to convert in to cosmos db expected format it gives me incorrect or may be correct date to update my database.

should I go ahead and update DB to ‘2021-08-19T18:30:00.000Z’ inplace of ‘8/20/2021’? or it should be ‘2021-08-20T00:00:00.000Z’?

please suggest