These notes refer to the war as defined by Great Britain.
1918
It is commonly believed that the war ended on 11 November 1918 but this is not the case. It was on that date, at 11am, that the armistice agreed with Germany came into effect and hostilities in France and Flanders were brought to a halt. It was a ceasefire only.
Hostilities had already ceased in most of the other theatres of war, but smaller operations were still continuing in North Russia, South Russia, Iraq and East Africa.
1919
On 28 June 1919, after protracted international discussions held mainly at Versailles, a peace treaty was signed between the Allied and Associated Powers (which included Great Britain) and Germany. It included provisions for defining the end of the war.
1920
On 9 February 1920, an order was issued from Buckingham Palace:
“And whereas by the said treaty of peace it was provided that a proces-verbal of the deposit of ratifications should be drawn up as soon as the treaty had been ratified by Germany on the one hand and by three of the principal Allied or Associated Powers on the other, and that from the date of the said proces-verbal the treaty would come into force between the high contracting parties who had ratified it: And whereas the said treaty having, been ratified by Germany and three of the principal Allied and Associated Powers, including His Majesty, such a proces-verbal as aforesaid has been drawn up dated the tenth day of January, nineteen hundred and twenty: And whereas treaties of peace with other belligerents not having yet been ratified it is desirable to declare the date which is to be treated as the date of the termination of war with Germany before declaring the date which is to be treated as the date of the termination of the present war: Now, therefore, His Majesty, by and with the advice of His Privy Council, is pleased to order, and it is hereby ordered, that the said tenth day of January shall be treated as the date of the termination of war between His Majesty and Germany.”
In ther words, war with Germany ended on 10 January 1920. Similar orders ended the war with Austria on 16 July 1920 and Bulgaria on 9 August 1920. War with Hungary similarly ended on 26 July 1921.
1921
Army Order 356 of 1921 stated that an Order in Council had been issued on 10 August 1921, which said that 31 August 1921 would be defined as the date which would be considered to be the legal termination of the war. That is, the day at midnight of which the war would be treated as ended.
Hostilities were still in progress with Turkey. The Army Order also explicitly said that its statement would not affect relations with Turkey until such time that a peace treaty had been exchanged or deposited.
Thanks to those who have contributed to discussion at the Great War Forum. This article was inspired by it and produced after checking sources quoted. Link