Covered here: https://wyday.com/forum/t/3906/able-to-extend-trial-days-by-changing-system-time/#post-18818
Summary: this is cooked up behavior that doesn't stand up to normal user-behavior (i.e. you can only "trick" TA under a very limited set of circumstances -- i.e. it's cooked up behavior). But we're "solving" this "problem" in TA 4.1 so we stop getting these "bug" reports.
Longer story, read here: https://wyday.com/forum/t/3906/able-to-extend-trial-days-by-changing-system-time/#post-18818