C# - How To Create A DateTime Object Not In The Current Culture?

I'm really drawing a blank on this one. I've been working on globalization but the DateTime seems to always revert back to the CurrentThread's culture.

I've got a textbox with the date expressed as a string:

// the CurrentThread's culture is de-DE
// My test browser is also set to de-DE
IFormatProvider culture = new System.Globalization.CultureInfo("de-DE",
true);
// en-US culture, what I'd ultimately like to see the DateTime in
IFormatProvider us_culture = new System.Globalization.CultureInfo("en-US",
true);
// correctly reads the textbox value (22.7.2010 into a datetime)
DateTime dt= DateTime.Parse(txtStartDate.Text, culture,
System.Globalization.DateTimeStyles.NoCurrentDateDefault);
// correctly produces a string 7/22/2010
string dt2 = dt.ToString(us_culture);

At this point I want a DateTime that's in en-US I've tried both:

DateTime dt3 = Convert.ToDateTime(dt2, us_culture);
DateTime dt3 = DateTime.Parse(dt2, us_culture);

But both produce de-DE DateTimes. My motivation in asking this question is the rest of the business logic is going to be calling dt2.toString() and will result in an incorrect date time string. I realize I could change toString() to be toString(us_culture) but I'd rather not change all of the rest of the business logic to accomodate this change.

Is there a way to get a DateTime in a culture other than the CurrentThread's culture?