Typescript undefined date validation

Question

I am using luxon for my app. I have some undefined date. I validate my date with luxon's isDateTime and function returns string. I used this function multiple places of the app. Most of the places I am passing date string and only places I am getting undefined date. I manage to silent the Typescript error but I think it's not optimal solution.

code-sandbox

Is there any better way, I can make this function?

const {
  DateTime
} = require('luxon')

const ISO_DATE_FORMAT = "yyyy-MM-dd";

const dateToStrings = (date: string | number | Date): DateTime =>
  DateTime.fromISO(new Date(date).toISOString());

const dateToISOString = (date: Date | string | undefined): string => { 
  if (DateTime.isDateTime(date)) {
    return dateToStrings(date).toFormat(ISO_DATE_FORMAT);
  }
  return date as string;
};

console.log(dateToISOString(undefined));