Add getDefaultStartDate utility method

- This computes the default start date
for the daterange picker

ee/app/assets/javascripts/analytics/productivity_analytics/index.js

 import Vue from 'vue';
 import { mapState, mapActions } from 'vuex';
-import { getDateInPast } from '~/lib/utils/datetime_utility';
 import { defaultDaysInPast } from './constants';
 import store from './store';
 import FilterDropdowns from './components/filter_dropdowns.vue';
 import DateRange from '../shared/components/daterange.vue';
 import ProductivityAnalyticsApp from './components/app.vue';
 import FilteredSearchProductivityAnalytics from './filtered_search_productivity_analytics';
-import { getLabelsEndpoint, getMilestonesEndpoint } from './utils';
+import { getLabelsEndpoint, getMilestonesEndpoint, getDefaultStartDate } from './utils';
 
 export default () => {
   const container = document.getElementById('js-productivity-analytics');
@@ -20,11 +19,11 @@ export default () => {
   const appContainer = container.querySelector('.js-productivity-analytics-app-container');
 
   const { endpoint, emptyStateSvgPath, noAccessSvgPath } = appContainer.dataset;
-  const { startDate: minDate } = timeframeContainer.dataset;
+  const { startDate: computedStartDate } = timeframeContainer.dataset;
 
-  const now = new Date(Date.now());
-  const defaultStartDate = getDateInPast(now, defaultDaysInPast);
-  const defaultEndDate = now;
+  const minDate = computedStartDate ? new Date(computedStartDate) : null;
+  const defaultStartDate = getDefaultStartDate(minDate, defaultDaysInPast);
+  const defaultEndDate = new Date(Date.now());
 
   let filterManager;
 
@@ -98,7 +97,7 @@ export default () => {
           show: this.groupNamespace !== null,
           startDate: defaultStartDate,
           endDate: defaultEndDate,
-          minDate: minDate ? new Date(minDate) : null,
+          minDate,
         },
         on: {
           change: this.onDateRangeChange,

ee/app/assets/javascripts/analytics/productivity_analytics/utils.js

@@ -32,6 +32,24 @@ export const getMilestonesEndpoint = (namespacePath, projectPathWithNamespace) =
   return `/groups/${namespacePath}/-/milestones`;
 };
 
+/**
+ * Determines the default min date for the productivity analytics date picker
+ * by taking the minDate (provided by the BE) into account.
+ * @param {Date} minDate - The min start date provided by the backend.
+ * @param {Number} defaultDaysInPast - The number of days in the past (used for computing the start date).
+ * @returns {Date} - The computed default start date.
+ */
+export const getDefaultStartDate = (minDate, defaultDaysInPast) => {
+  const now = new Date(Date.now());
+  const dateInPast = getDateInPast(now, defaultDaysInPast);
+
+  if (!minDate) {
+    return dateInPast;
+  }
+
+  return minDate > dateInPast ? minDate : dateInPast;
+};
+
 /**
  * Computes the day difference 'days' between a given start and end date
  * and creates an array of length 'days'.
@@ -64,9 +82,9 @@ export const initDateArray = (startDate, endDate) => {
  *   [{ merged_at: '2019-09-03T21:29:49.351Z', metric: 24 }, ... ] // 2019-09-03
  * ]
  *
- * @param {*} data - The raw data received from the API.
- * @param {*} startDate - The start date selected by the user minus an additional offset in days (e.g., 30 days).
- * @param {*} endDate - The end date selected by the user.
+ * @param {Object} data - The raw data received from the API.
+ * @param {Date} startDate - The start date selected by the user minus an additional offset in days (e.g., 30 days).
+ * @param {Date} endDate - The end date selected by the user.
  * @returns {Array} The transformed data array (first item corresponds to start date, last item to end date)
  */
 export const transformScatterData = (data, startDate, endDate) => {
@@ -108,9 +126,9 @@ export const transformScatterData = (data, startDate, endDate) => {
  * i[1] = metric, displayed on y axis
  * i[2] = datetime, used in the tooltip
  *
- * @param {*} data - The already transformed scatterplot data (which is computed by transformScatterData)
- * @param {*} startDate - The start date selected by the user
- * @param {*} endDate - The end date selected by the user
+ * @param {Array} data - The already transformed scatterplot data (which is computed by transformScatterData)
+ * @param {Date} startDate - The start date selected by the user
+ * @param {Date} endDate - The end date selected by the user
  * @returns {Array} An array with each item being another arry of two items (date, computed median)
  */
 export const getScatterPlotData = (data, startDate, endDate) => {
@@ -139,9 +157,9 @@ export const getScatterPlotData = (data, startDate, endDate) => {
  * Example: Rolling median for m days:
  * Calculate median for day i: median[i - m + 1 ... i]
  *
- * @param {*} data - The already transformed scatterplot data (which is computed by transformScatterData)
- * @param {*} startDate - The start date selected by the user
- * @param {*} endDate - The end date selected by the user
+ * @param {Array} data - The already transformed scatterplot data (which is computed by transformScatterData)
+ * @param {Date} startDate - The start date selected by the user
+ * @param {Date} endDate - The end date selected by the user
  * @param {Number} daysOffset The number of days that to look up data in the past (e.g. 30 days in the past for 30 day rolling median)
  * @returns {Array} An array with each item being another arry of two items (date, computed median)
  */

ee/spec/frontend/analytics/productivity_analytics/utils_spec.js

 import {
   getLabelsEndpoint,
   getMilestonesEndpoint,
+  getDefaultStartDate,
   initDateArray,
   transformScatterData,
   getScatterPlotData,
@@ -35,6 +36,32 @@ describe('Productivity Analytics utils', () => {
     });
   });
 
+  describe('getDefaultStartDate', () => {
+    const realDateNow = Date.now;
+    const defaultDaysInPast = 10;
+
+    beforeAll(() => {
+      const today = jest.fn(() => new Date('2019-10-01'));
+      global.Date.now = today;
+    });
+
+    afterAll(() => {
+      global.Date.now = realDateNow;
+    });
+
+    it('returns the minDate when the computed date (today minus defaultDaysInPast) is before the minDate', () => {
+      const minDate = new Date('2019-09-30');
+
+      expect(getDefaultStartDate(minDate, defaultDaysInPast)).toEqual(minDate);
+    });
+
+    it('returns the computed date (today minus defaultDaysInPast) when this is after the minDate', () => {
+      const minDate = new Date('2019-09-01');
+
+      expect(getDefaultStartDate(minDate, defaultDaysInPast)).toEqual(new Date('2019-09-21'));
+    });
+  });
+
   describe('initDateArray', () => {
     it('creates a two-dimensional array with 3 empty arrays for startDate=2019-09-01 and endDate=2019-09-03', () => {
       const startDate = new Date('2019-09-01');