Angular Date Validator - directive and factory - validates strings m/d/yyyy

formUtils.factory.ts

// MM/DD/YYYY, M/D/YYYY
const DATE_REGEX = new RegExp(/^(\d{2}|\d{1})\/(\d{2}|\d{1})\/\d{4}$/);

export { DATE_REGEX };

validateDate.component.html

<input
  id="endDate"
  name="endDate"
  type="text"
  class="form-control"
  #endDate="ngModel"
  appDateField
  required
  [(ngModel)]="form.endDate">

validateDate.directive.ts

import { Directive } from '@angular/core';
import { NG_VALIDATORS, Validator, ValidatorFn, FormControl } from '@angular/forms';
import { dateValidator } from './validateDate.factory';

@Directive({
  selector: '[appDateField]',
  providers: [
    { provide: NG_VALIDATORS, useExisting: ValidateDateDirective, multi: true }
  ]
})
export class ValidateDateDirective implements Validator {
  private _valFn: ValidatorFn;

  constructor() {
    this._valFn = dateValidator();
  }

  validate(control: FormControl) {
    return this._valFn(control);
  }

}

validateDate.factory.ts

import { AbstractControl, ValidatorFn } from '@angular/forms';
import { DATE_REGEX } from './formUtils.factory';

export function dateValidator(): ValidatorFn {
  return (control: AbstractControl): {[key: string]: any} => {
    const dateStr = control.value;
    // Length of months (will update for leap years)
    const monthLength = [ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 ];
    // Object to return if date is invalid
    const invalidObj = { 'date': true };

    // First check for m/d/yyyy or mm/dd/yyyy format
    // If the pattern is wrong, don't validate dates yet
    if (!DATE_REGEX.test(dateStr)) {
      return null;
    }

    // Parse the date input to integers
    const parts = dateStr.split('/');
    const month = parseInt(parts[0], 10);
    const day = parseInt(parts[1], 10);
    const year = parseInt(parts[2], 10);

    // Make sure date is in range
    if (year < 2000 || year > 3000 || month === 0 || month > 12) {
      return invalidObj;
    }
    // Adjust for leap years
    if (year % 400 === 0 || (year % 100 !== 0 && year % 4 === 0)) {
      monthLength[1] = 29;
    }
    // Check the range of the day
    if (!(day > 0 && day <= monthLength[month - 1])) {
      return invalidObj;
    };
    
    return null;
  };
}