Add explanation about maxStartDate and minEndDate,

Refactor calc max and min functions

js/components/__tests__/pop_date_range.test.js

@@ -8,6 +8,12 @@ import { makeTestWrapper } from '../../test_utils/component_test_helpers'
 const PopDateRangeWrapper = makeTestWrapper({
   components: { PopDateRange },
   templatePath: 'pop_date_range.html',
+  data: function() {
+    return {
+      initialMinStartDate: '2019-09-14',
+      initialMaxEndDate: '2022-09-14',
+    }
+  },
 })
 
 describe('PopDateRange Test', () => {