app.component.ts

  /**
   * An example of custom validator on a date string.
   * Valid date is a string, which is:
   * 1, in the form of YYYY-MM-DD
   * 2, later than today
   * 3, not an invalid value like 2018-20-81
   * @param date a date string
   */
  isMyDateFormat(date: string): string {
    if (date.length !== 10) {
      return 'Invalid input: Please input a string in the form of YYYY-MM-DD';
    } else {
      const da = date.split('-');
      if (da.length !== 3 || da[0].length !== 4 || da[1].length !== 2 || da[2].length !== 2) {
        return 'Invalid input: Please input a string in the form of YYYY-MM-DD';
      } else if (moment(date).isValid()) {
        return 'Invalid date: Please input a date no later than today';
      } else if (!moment(date).isValid()) {
        return 'Invalid date: Please input a date with a valid month and date.';
      }
    }
    return 'Unknown error.';
  }