Today I learned that the office of Prime Minister of the United Kingdom took its modern form 300 years ago this month, when Sir Robert Walpole took office as First Lord of the Treasury on 4 April 1721.
Of course, this being the UK, governed more by tradition and custom than a founding document like nearly every other country on Earth, it gets a bit fuzzier on investigation. The office of First Lord of the Treasury dates back to 1126, when King Henry I appointed Nigel, Bishop of Ely, his Lord High Treasurer. The office morphed into First Lord of the Treasury in 1714 when Charles Montagu, First Earl Halifax, assumed the post.
But when Walpole took the brief for the second time in 1721 (he also held the post from 1715 to 1717), King George I granted him emergency powers to stabilize the country after the South Sea Company's collapse in 1720. He not only handled the emergency, but he also managed to make the office part of the constitutional framework of UK governance. It would take until 1937 for UK law to recognize the office of Prime Minister formally.