What does within 20 calendar days mean?

When the term "within" is used relative to time, it typically means before the expiration of, not beyond or not later than. Thus, if an answer is required "within 20 days of July 1", submission on the 20th would be sufficient but late on the 21st. See Glenn v. Garrett, Tex.Civ.App., 84 S.W.2d 515, 516.