A week is a long time in politics but a short stretch in political history. The seven days that have passed between Tony Blair’s interview with The Times and the aftershocks of yesterday have been extraordinary. Yet, on the other hand, they have hardly been unprecedented. They have largely served to show that the old rules of British politics still apply in a modern context and that our unusual, allegedly antiquated, unwritten constitution is more robust than regulations set out on paper.

For what this explosive episode has demonstrated again is that the great arbiter in the British system of government is time. It is time, more than anything else, that has closed in on Mr Blair. There is a natural limit to prime ministerial tenures in Britain, which, in reality, is as restrictive on an incumbent as the 22nd Amendment to the US Constitution (which bars a president from being elected more than twice). Time, and relentless, wearying scrutiny ensure in Britain that a party and the public can only endure so much of any prime minister, no matter how charming or how determined.

Time is, above all else, what counts against this Prime Minister. After nine years in office, new speeches and initiatives inevitably seem stale, colleagues yearn for a change (perhaps unwisely) and the cadre of enemies expands to overtake the collection of remaining friends. Mr Blair was always opposed by the ultra-Left of his party (a factor that reflects well on him) but time has meant that its ranks have been swollen by those whom he first hired then fired and by others whom he never promoted. New rounds of MPs enter Parliament with their own perspectives. Time is the knock on the door in the middle of the night that must be answered. Time has made its unavoid-able call on Downing Street.

There has been much chatter about “presidentialism” in Britain’s political life and how Mr Blair has marginalised the Government and the House of Commons. What this week has surely illustrated is how facile that generalisation is in practice. The Prime Minister was obliged to compromise on his resignation date by resignations from an unknown junior minister and seven anonymous aides, and the threat that others would follow them. The traditional norms apply. A prime minister must retain the backing of his MPs and party to survive in power. Britain is not a permanent elective dictatorship. Ours is not, despite all the hype, a de facto “presidential” model.

Whether there will be a stable and orderly transition from the so-called President Blair to the so-called President Brown should become clearer over the next week. Mr Brown is closer to the crown than ever but the chaos and factionalism of the past week has made it acceptable for his opponents to begin to make statements about him in public that they previously would only ever have whispered in private (and then deny having made).

Charles Clarke, the former Home Secretary, has obviously been deeply irritated by Mr Brown’s unpresidential, unguarded, “stupid” smile, though it is well known that Mr Clarke has difficulty keeping impulsive thoughts to himself. What should be of more concern to the Chancellor is the prospect of the engaging Education Secretary, Alan Johnson, standing against him for the leadership. If Mr Brown does not pass the presidential personality test (political generosity being among the questions), Mr Johnson will have no choice but to run as well.