Edward VIII

If you're counting the actual number of days (which we are), this depends on when you consider the abdication to have taken effect.

King George V was certified dead at 11:55 pm on 20 January 1936. According to Britannica, "Edward renounced the British throne on December 10, 1936 (confirmed by the Declaration of Abdication Act the following day)". Britannica dates the reign to 10 December. But Wikipedia gives the dates of Edward VIII's reign as 20 January to 11 December: "Edward ... signed the instruments of abdication at Fort Belvedere on 10 December 1936" ... [but] the next day, the last act of his reign was the royal assent to His Majesty's Declaration of Abdication Act 1936.

In counting the days, I think we can probably assume that you include either the date of accession or the date of the abdication, but not both; let's say we include the date of the abdication. This means we discount 20 January, so we count 11 days in January and either 10 or 11 in December; total 21 or 22. The ten months from February to November total 304 days (5 x 31, 4 x 30, and 1 x 29 – 1936 being a leap year). So that's a total of 325 days according to Britannica, but 326 according to Wikipedia.

The Royal Family's own website sides with Britannica: "On 10 December 1936, Edward VIII executed an Instrument of Abdication which was given legal effect the following day ... his reign lasted only 325 days." But surely Edward was still the King until the abdication was "given legal effect"? Otherwise, how could he have given royal assent?

For what it's worth, my 1980 copy of the Guinness Book of Answers agrees with Wikipedia – dating Edward's reign to 11 December 1936.