_getStartOfCurrentDay() {
        const timezone = this._settingsCache.get('timezone') || 'Etc/UTC';
        const startOfDay = moment().tz(timezone).startOf('day').utc().toISOString();
        return startOfDay;
    }
Returns the start of the current day in the site timezone
@returns {string} The start of the current day in the site timezone, formatted as a ISO 8601 string