REFACTOR: local dates to improve reliability with DST and recurrence (#9379)

This commit improves testing and separates local-date generation from dates with zone manipulations.

plugins/discourse-local-dates/assets/javascripts/lib/date-with-zone-helper.js.es6

@@ -0,0 +1,89 @@
+const { getProperties } = Ember;
+
+/*
+  DateWithZoneHelper provides a limited list of helpers
+  to manipulate a moment object with timezones
+
+  - add(count unit) adds a COUNT of UNITS to a date
+  - subtract(count unit) subtracts a COUNT of UNITS to a date
+  - format(format) formats a date with zone in a consitent way, optional moment format
+  - isDST() allows to know if a date in a specified timezone is currently under DST
+  - datetimeWithZone(timezone) returns a new moment object with timezone applied
+  - datetime returns the moment object
+  - repetitionsBetweenDates(duration, date) return the number of repertitions of
+  duration between two dates, eg for duration: "1.weeks", "2.months"...
+*/
+export default class DateWithZoneHelper {
+  constructor(params = {}) {
+    this.timezone = params.timezone || "UTC";
+    this.localTimezone = params.localTimezone || moment.tz.guess();
+
+    this.datetime = moment.tz(
+      getProperties(params, [
+        "year",
+        "month",
+        "day",
+        "hour",
+        "minute",
+        "second"
+      ]),
+      this.timezone
+    );
+  }
+
+  isDST() {
+    return this.datetime.tz(this.localTimezone).isDST();
+  }
+
+  repetitionsBetweenDates(duration, date) {
+    const [count, unit] = duration.split(".");
+    const diff = this.datetime.diff(date, unit);
+    const repetitions = diff / parseInt(count, 10);
+    return Math.abs((Math.round(repetitions * 10) / 10).toFixed(1));
+  }
+
+  add(count, unit) {
+    return this._fromDatetime(
+      this.datetime.clone().add(count, unit),
+      this.timezone,
+      this.localTimezone
+    );
+  }
+
+  subtract(count, unit) {
+    return this._fromDatetime(
+      this.datetime.clone().subtract(count, unit),
+      this.timezone,
+      this.localTimezone
+    );
+  }
+
+  datetimeWithZone(timezone) {
+    return this.datetime.clone().tz(timezone);
+  }
+
+  format(format) {
+    if (format) {
+      return this.datetime.tz(this.localTimezone).format(format);
+    }
+
+    return this.datetime.tz(this.localTimezone).toISOString(true);
+  }
+
+  static fromDatetime(datetime, timezone, localTimezone) {
+    return new DateWithZoneHelper({
+      year: datetime.year(),
+      month: datetime.month(),
+      day: datetime.date(),
+      hour: datetime.hour(),
+      minute: datetime.minute(),
+      second: datetime.second(),
+      timezone,
+      localTimezone
+    });
+  }
+
+  _fromDatetime(datetime, timezone, localTimezone) {
+    return DateWithZoneHelper.fromDatetime(datetime, timezone, localTimezone);
+  }
+}