Fixed time difference calculation in DST timezones

refs https://github.com/TryGhost/Team/issues/588

- date-fns proved to be unable to manipulate dates consistently in UTC timezone. Keeping all calculations and formatting in UTC is key to have consistency in dates when dealing in inter-system dates
- day.js also failed the test for correct UTC manipulation. See https://github.com/iamkun/dayjs/issues/1271 for example bug which prevents from consistent correct calculation
- luxon was the best option which WORKED. It's also a recommended successor for moment.js with really nice docs and active support

ghost/limit-service/lib/date-utils.js

@@ -1,6 +1,4 @@
-const differenceInMonths = require('date-fns/differenceInMonths');
-const parseISO = require('date-fns/parseISO');
-const addMonths = require('date-fns/addMonths');
+const {DateTime} = require('luxon');
 
 const SUPPORTED_INTERVALS = ['month'];
 /**
@@ -14,11 +12,13 @@ const SUPPORTED_INTERVALS = ['month'];
  */
 const lastPeriodStart = (startDate, interval) => {
     if (interval === 'month') {
-        const startDateISO = parseISO(startDate);
-        const fullPeriodsPast = differenceInMonths(new Date(), startDateISO);
-        const lastPeriodStartDate = addMonths(startDateISO, fullPeriodsPast);
+        const startDateISO = DateTime.fromISO(startDate, {zone: 'UTC'});
+        const now = DateTime.now().setZone('UTC');
+        const fullPeriodsPast = Math.floor(now.diff(startDateISO, 'months').months);
 
-        return lastPeriodStartDate.toISOString();
+        const lastPeriodStartDate = startDateISO.plus({months: fullPeriodsPast});
+
+        return lastPeriodStartDate.toISO();
     }
 
     throw new Error('Invalid interval specified. Only "month" value is accepted.');

ghost/limit-service/package.json

@@ -26,6 +26,7 @@
     "sinon": "10.0.0"
   },
   "dependencies": {
-    "lodash": "^4.17.21"
+    "lodash": "^4.17.21",
+    "luxon": "^1.26.0"
   }
 }