DateTime conversion not working from Thai to English

I have an application where users can select language and current culture will be set for that language. I an facing issues when users select thai language. Since thai calendar is 543 years ahead from georgian calendar, date shows as 2017+543. This is fine. But the problem is when I try to convert date from thai culture to english culture, somehow it adds more 543 years. Means currently it's 2017+543 = 2560. It adds more 543 years 2560 + 543= 3103. Can someone help me please. Please note currentthread.currentculture will be thai Here is a code snippet

DateTime dt = DateTime.Now;
String format = "MM/dd/yyyy hh:mm:ss";

System.Globalization.CultureInfo us = new System.Globalization.CultureInfo("en-US");
IFormatProvider culture = System.Globalization.CultureInfo.CreateSpecificCulture("en-US");
String strDate = dt.ToString(format);

DateTime serverSysdate = DateTime.ParseExact(strDate, "MM/dd/yyyy hh:mm:ss", culture);