var rates = client.getTable('exchangeRates');
        rates.where(function (cur, low, high) {
            return this.currency == cur && this.time >= low && this.time <= high;
        }, cur, low, high).orderBy("time").read().done(function (results) {
            //no currency rate found on given date range
            if (results.length == 0) {
                //something went wrong
            }
            else {
                //results[0].date is the given date
                callback(results);
            }
        }, function (err) {
            app.showNotification("Error: " + err);
        });