The words which and that have no possessive form. Here the of which is showing the state of the statute possessing a purpose. We cannot say, that’s purpose or which’s purpose. We have to use which, flip it over, and connect it to statute by using the of which form. The word that will not accommodate a preceding preposition.
Of course, if you write lots of reports crammed with lots of of whiches, you won’t further your career. So Miss Hamrick, Igor, and Amber made a deal with the personal relative pronouns. The who-whom-whose combo, with its possessive whose, agreed to allow that and which to borrow whose when they needed to show possession. Thus, it is grammatically correct to write:
It was an idea the time of which had come.
It was an idea whose time had come.