I am building a component (html, css, spec.ts, ts) in angular in which I always want endDate > startDate. I have followed this link https://material.angular.io/components/datepi
Unless I'm missing something, you can use the [min]="" date property:
Whatever startDate is will limit the Calendar dates available for endDate. If you need it set before startDate is chosen, use another variable and set it in consturctor() or ngOnInit() of your component.
See: https://material.angular.io/components/datepicker/overview#date-validation