WinRT CultureInfo and Calender

Question

I am developing a multilingual WinRT C# app.

There are 2 languages supported, respectively Arabic and English.

when i Set Arabic mode in my application

System.Globalization.CultureInfo.DefaultThreadCurrentUICulture = new CultureInfo("ar");

Now my application adopts to Arabic perfectly(using proper Resources, formatting , etc), but when is set TimeFormat (eg: DateTime.ParseExact(dataValue, "yyyyMMdd", null);)FormatExceptionOccered stating

The DateTime represented by the string is not supported in calendar System.Globalization.UmAlQuraCalendar.

I want all the Date time Format to be presented in Default(English Mode)Mode.