Add date time format utilities

This adds a few date time formatting utils to be used in the DAST
on-demand scans scheduling options.

* isoDateToSameLocaleDate will be used to reset the date and time inputs
  to the correct values when initializing the form with a stored
  datetime and timezone.
* dateAndTimeToUTCString will let us compute a Date instance based on a
  Date generated by a datepicker and a time string from a time input.
* dateToTimeInputValue will serve to restore a time input's value from a
  Date instance.

app/assets/javascripts/lib/utils/datetime/date_format_utility.js

 import dateFormat from 'dateformat';
-import { isString, mapValues, reduce } from 'lodash';
+import { isString, mapValues, reduce, isDate } from 'lodash';
 import { s__, n__, __ } from '../../../locale';
 
 /**
@@ -258,3 +258,100 @@ export const parseSeconds = (
     return periodCount;
   });
 };
+
+/**
+ * Pads given items with zeros to reach a length of 2 characters.
+ *
+ * @param  {...any} args Items to be padded.
+ * @returns {Array<String>} Padded items.
+ */
+export const padWithZeros = (...args) => args.map((arg) => `${arg}`.padStart(2, '0'));
+
+/**
+ * This removes the timezone from an ISO date string.
+ * This can be useful when populating date/time fields along with a distinct timezone selector, in
+ * which case we'd want to ignore the timezone's offset when populating the date and time.
+ *
+ * Examples:
+ * stripTimezoneFromISODate('2021-08-16T00:00:00.000-02:00') => '2021-08-16T00:00:00.000'
+ * stripTimezoneFromISODate('2021-08-16T00:00:00.000Z') => '2021-08-16T00:00:00.000'
+ *
+ * @param {String} date The ISO date string representation.
+ * @returns {String} The ISO date string without the timezone.
+ */
+export const stripTimezoneFromISODate = (date) => {
+  if (Number.isNaN(Date.parse(date))) {
+    return null;
+  }
+  return date.replace(/(Z|[+-]\d{2}:\d{2})$/, '');
+};
+
+/**
+ * Extracts the year, month and day from a Date instance and returns them in an object.
+ * For example:
+ * dateToYearMonthDate(new Date('2021-08-16')) => { year: '2021', month: '08', day: '16' }
+ *
+ * @param {Date} date The date to be parsed
+ * @returns {Object} An object containing the extracted year, month and day.
+ */
+export const dateToYearMonthDate = (date) => {
+  if (!isDate(date)) {
+    // eslint-disable-next-line @gitlab/require-i18n-strings
+    throw new Error('Argument should be a Date instance');
+  }
+  const [year, month, day] = date.toISOString().replace(/T.*$/, '').split('-');
+  return { year, month, day };
+};
+
+/**
+ * Extracts the hours and minutes from a string representing a time.
+ * For example:
+ * timeToHoursMinutes('12:46') => { hours: '12', minutes: '46' }
+ *
+ * @param {String} time The time to be parsed in the form HH:MM.
+ * @returns {Object} An object containing the hours and minutes.
+ */
+export const timeToHoursMinutes = (time = '') => {
+  if (!time || !time.match(/\d{1,2}:\d{1,2}/)) {
+    // eslint-disable-next-line @gitlab/require-i18n-strings
+    throw new Error('Invalid time provided');
+  }
+  const [hours, minutes] = padWithZeros(...time.split(':'));
+  return { hours, minutes };
+};
+
+/**
+ * This combines a date and a time and returns the computed Date's ISO string representation.
+ *
+ * @param {Date} date Date object representing the base date.
+ * @param {String} time String representing the time to be used, in the form HH:MM.
+ * @param {String} offset An optional Date-compatible offset.
+ * @returns {String} The combined Date's ISO string representation.
+ */
+export const dateAndTimeToUTCString = (date, time, offset = '') => {
+  const { year, month, day } = dateToYearMonthDate(date);
+  const { hours, minutes } = timeToHoursMinutes(time);
+
+  return new Date(
+    `${year}-${month}-${day}T${hours}:${minutes}:00.000${offset || 'Z'}`,
+  ).toISOString();
+};
+
+/**
+ * Converts a Date instance to time input-compatible value consisting in a 2-digits hours and
+ * minutes, separated by a semi-colon, in the 24-hours format.
+ *
+ * @param {Date} date Date to be converted
+ * @returns {String} time input-compatible string in the form HH:MM.
+ */
+export const dateToTimeInputValue = (date) => {
+  if (!isDate(date)) {
+    // eslint-disable-next-line @gitlab/require-i18n-strings
+    throw new Error('Argument should be a Date instance');
+  }
+  return date.toLocaleTimeString([], {
+    hour: '2-digit',
+    minute: '2-digit',
+    hour12: false,
+  });
+};

spec/frontend/lib/utils/datetime/date_format_utility_spec.js

+import * as utils from '~/lib/utils/datetime/date_format_utility';
+
+describe('date_format_utility.js', () => {
+  describe('padWithZeros', () => {
+    it.each`
+      input    | output
+      ${0}     | ${'00'}
+      ${'1'}   | ${'01'}
+      ${'10'}  | ${'10'}
+      ${'100'} | ${'100'}
+      ${100}   | ${'100'}
+      ${'a'}   | ${'0a'}
+      ${'foo'} | ${'foo'}
+    `('properly pads $input to match $output', ({ input, output }) => {
+      expect(utils.padWithZeros(input)).toEqual([output]);
+    });
+
+    it('accepts multiple arguments', () => {
+      expect(utils.padWithZeros(1, '2', 3)).toEqual(['01', '02', '03']);
+    });
+
+    it('returns an empty array provided no argument', () => {
+      expect(utils.padWithZeros()).toEqual([]);
+    });
+  });
+
+  describe('stripTimezoneFromISODate', () => {
+    it.each`
+      input                          | expectedOutput
+      ${'2021-08-16T00:00:00Z'}      | ${'2021-08-16T00:00:00'}
+      ${'2021-08-16T10:30:00+02:00'} | ${'2021-08-16T10:30:00'}
+      ${'2021-08-16T10:30:00-05:30'} | ${'2021-08-16T10:30:00'}
+    `('returns $expectedOutput when given $input', ({ input, expectedOutput }) => {
+      expect(utils.stripTimezoneFromISODate(input)).toBe(expectedOutput);
+    });
+
+    it('returns null if date is invalid', () => {
+      expect(utils.stripTimezoneFromISODate('Invalid date')).toBe(null);
+    });
+  });
+
+  describe('dateToYearMonthDate', () => {
+    it.each`
+      date                      | expectedOutput
+      ${new Date('2021-08-05')} | ${{ year: '2021', month: '08', day: '05' }}
+      ${new Date('2021-12-24')} | ${{ year: '2021', month: '12', day: '24' }}
+    `('returns $expectedOutput provided $date', ({ date, expectedOutput }) => {
+      expect(utils.dateToYearMonthDate(date)).toEqual(expectedOutput);
+    });
+
+    it('throws provided an invalid date', () => {
+      expect(() => utils.dateToYearMonthDate('Invalid date')).toThrow(
+        'Argument should be a Date instance',
+      );
+    });
+  });
+
+  describe('timeToHoursMinutes', () => {
+    it.each`
+      time       | expectedOutput
+      ${'23:12'} | ${{ hours: '23', minutes: '12' }}
+      ${'23:12'} | ${{ hours: '23', minutes: '12' }}
+    `('returns $expectedOutput provided $time', ({ time, expectedOutput }) => {
+      expect(utils.timeToHoursMinutes(time)).toEqual(expectedOutput);
+    });
+
+    it('throws provided an invalid time', () => {
+      expect(() => utils.timeToHoursMinutes('Invalid time')).toThrow('Invalid time provided');
+    });
+  });
+
+  describe('dateAndTimeToUTCString', () => {
+    it('computes the date properly', () => {
+      expect(utils.dateAndTimeToUTCString(new Date('2021-08-16'), '10:00')).toBe(
+        '2021-08-16T10:00:00.000Z',
+      );
+    });
+
+    it('computes the date properly with an offset', () => {
+      expect(utils.dateAndTimeToUTCString(new Date('2021-08-16'), '10:00', '-04:00')).toBe(
+        '2021-08-16T14:00:00.000Z',
+      );
+    });
+
+    it('throws if date in invalid', () => {
+      expect(() => utils.dateAndTimeToUTCString('Invalid date', '10:00')).toThrow(
+        'Argument should be a Date instance',
+      );
+    });
+
+    it('throws if time in invalid', () => {
+      expect(() => utils.dateAndTimeToUTCString(new Date('2021-08-16'), '')).toThrow(
+        'Invalid time provided',
+      );
+    });
+
+    it('throws if offset is invalid', () => {
+      expect(() =>
+        utils.dateAndTimeToUTCString(new Date('2021-08-16'), '10:00', 'not an offset'),
+      ).toThrow('Invalid time value');
+    });
+  });
+
+  describe('dateToTimeInputValue', () => {
+    it.each`
+      input                                        | expectedOutput
+      ${new Date('2021-08-16T10:00:00.000Z')}      | ${'10:00'}
+      ${new Date('2021-08-16T22:30:00.000Z')}      | ${'22:30'}
+      ${new Date('2021-08-16T22:30:00.000-03:00')} | ${'01:30'}
+    `('extracts $expectedOutput out of $input', ({ input, expectedOutput }) => {
+      expect(utils.dateToTimeInputValue(input)).toBe(expectedOutput);
+    });
+
+    it('throws if date is invalid', () => {
+      expect(() => utils.dateToTimeInputValue('Invalid date')).toThrow(
+        'Argument should be a Date instance',
+      );
+    });
+  });
+});