If you mean adding two dates this will help
System.DateTime today = System.DateTime.Now;
System.TimeSpan duration = new System.TimeSpan(36, 0, 0, 0);
System.DateTime answer = today.Add(duration);
System.Console.WriteLine("{0:dddd}", answer);
you can find more details about this in links below
TimeSpan Structure[
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DateTime.Add Method [
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