var result = DateTime.Now.AddDays(-26 * 7);
var result2 = DateTime.Now.Add(new TimeSpan(-26 * 7, 0, 0, 0));
var result3 = DateTime.Now.AddHours(-26 * 7 * 24);
var result4 = DateTime.Now.Subtract(new TimeSpan(26 * 7, 0, 0, 0));
var result5 = DateTime.Now.Subtract(new TimeSpan(0, 26 * 7 * 24, 0, 0));
И так одно.