Fixed picker for local tz use (#21658)

ref 47b8161805

This ended up inverting the behavior, such that TZs far in advance of
GMT fouled up. This change builds the date by date components in the
local TZ so we should not run into further trouble...

ghost/admin/app/components/gh-date-time-picker.js

@@ -53,7 +53,10 @@ export default class GhDateTimePicker extends Component {
     
     @computed('_date')
     get localDateValue() {
-        return this._date.toDate();
+        // extract parts so we can build a date object that is strictly the day in the local tz
+        const [year, month, day] = this._date.format(DATE_FORMAT).split('-').map(Number);
+        const date = new Date(year, month - 1, day); // month is 0-indexed in the js date object
+        return date;
     }
 
     @computed('blogTimezone')