A secret group of environmentalist ultra-extremists has engineered a virus (or virus-like pathogen) with the purpose of making all of humanity extinct (yes, including themselves). They are aware that humanity will fight back against this plague as soon as they notice lots of people dying. There would undoubtedly be efforts to create vaccines, quarantines, and maybe even long term sealed underground shelters.
To avoid all of this, the virus was engineered to initially be mistaken for a mild cold, so it would have time to spread. A newly infected person would experience one or two days of mild sneezing and sniffles, after which the virus would go dormant. During this time, they would spread the virus to everyone they had contact with. After several years of this, the virus should be able to infect 100% of the world's population without anyone noticing.
Then, on a specific predetermined day and time throughout the world, the virus suddenly would switch to "deadly mode". Every person on Earth would become horrifically ill at the same time. There can be no emergency response, no medical care, no quarantines, because all of the people who would do such things are too busy dying. Within 24 hours, everyone is dead.
My question is, by what mechanism could this super-virus suddenly become deadly? How could a virus (or other pathogen designed to be mistaken for a virus) change its behavior at a predetermined time? It seems like the possibilities fall into two broad categories: internal mechanisms (each virus particle has something inside it that acts like a clock), or external mechanisms (the virus can detect something in the environment that happens on a given date).
Note that my question is not "would this succeed in making humanity extinct?" (I personally believe it would not. This is the basis of my story, actually.)
My question is also not "Are there more effective ways to achieve this?" (Undoubtedly there are. This plan was created by crazy extremists, and they chose to do it this way for their own reasons.)