To elaborate, monotonicity means that a value is either always increasing or always decreasing, and never changes direction. If a program is built on the assumption that time always goes forward, it may not like you setting the clock back 0.000000001sec every hour.
Very true, a lot of software with time limited trials or features like "check out a network license for 30 days of offline use" used to be easy to bypass by turning your clock back so that it never expired.
This is pretty widely fixed now, but often overzealously. Clock changed unexpectedly? User is a dirty pirate and the license should deactivate.
