Pass Validity

Would I require a 21-day JRPass as I am in Japan for 20-days?

I am not travelling every day so does the JRPass work on the number of days of travel (I will be travelling on 10-days so would a 7-day pass or 14-day pass be better.

So the question is really, does the Pass number of days (7,14,21) refer to continuous calendar days (e.g. 1st to 7th) or the number of days travelling e.g. I will travel roughly on the 9th,13th, 15th,18th,20th,24th and 28th.

Thank you