Mr. Anderson wrote:
Nicholas the Italian wrote:
date between yesterday and someday in the future.
In that case we have 100% guarantee

Not necessarily. All the computers that have to code on could have a catastrophic failure, every single one of them, and then any backups anyone has could suddenly also fail, loosing every single copy of the code. The devs could then keel over and die from shock, as could everyone else with the knowledge and wilingness to start on the project all over again.
I admit that it's not very likely, but, it'd stop us having 100%. Maybe 100% (to 3sf). But, not truly 100%.