Don't assume any format: just use the current format and that should be what the user entered (since that's what he likes to use):
public bool DateValidation(string stringDateValue)
{
DateTime d;
return DateTime.TryParse(stringDateValue, out d);
}
Or better still, use a DateTimePicker and you don't need to validate it as a valid DateTime as the user can't enter a "bad" one.