Is this what you excepted?
var ar = DateTime.Now.ToString("dd/MM/yy HH:mm:ss tt",
new System.Globalization.CultureInfo("ar-AE"));
// 14/03/14 06:14:34 ص
var us = DateTime.Now.ToString("dd/MM/yy HH:mm:ss tt",
new System.Globalization.CultureInfo("en-US"));
// 14/03/14 06:14:44 AM
--Edit
And you can get the datetime instance by DateTime.ParseExact :
var datetime = DateTime.ParseExact(us,"dd/MM/yy HH:mm:ss tt",
new System.Globalization.CultureInfo("en-US"));