If you want to change the culture of your entire application then you can set the culture when the application starts like this:
Thread.CurrentThread.CurrentCulture = new CultureInfo("en-US", false);
You can read about setting it on MSDN.
Given your example though, that may be overkill. If you are trying to change just the ToShortDateString() to en-US there may be a more simple way. You can use the ToString() method instead and pass in a specific format, for example you can do:
aDateTime.ToString("MM/dd/yyyy");
More specifically you can use the pre-defined culture info with System.Globalization like this:
System.Globalization.CultureInfo culture = new System.Globalization.CultureInfo("en-US");
string datePattern = culture.DateTimeFormat.ShortDatePattern;
string shortDate = aDateTime.ToString(datePattern);