This is where the classification heuristics and fine-grained permissions come in. Code that does anything conditioned on dates is higher risk. Code that uses location services is higher risk.

Also, many of these checks can be done by the browser in situ, so an extension that suddenly changes its behavior can be flagged for review. And pre-release malware scans can be run on banks of actual hardware that simulates different dates and locations.

Sure, there will be an arms race, but that's better than an anarchical free for all.