For World War Two, the conventional start and end dates (1st September 1939 and 2nd September 1945 respectively) are generally accepted, but (a) there is room for some minor quibbling about the exact dates and (b) some quite different dates can be reasonably suggested, and are sometimes suggested.

The conventional starting date is 1st September 1939, when Germany invaded Poland. A war between Germany and Poland is hardly a "world war", so one could say that WW2 started 3rd September, when Britain (and India, Australia, and New Zealand) and France entered the war. (More of the Commonwealth joined in over the next week: Newfoundland (which was not yet part of Canada) on the 4th, South Africa on the 6th, and Canada on the 10th).

The end dates for the war in Europe and the war as a whole are fuzzier. For WWII in Europe, there are two conventional end dates: for the West, VE Day is 8th May, while for the Soviet Union and Russia, the Great Patriotic War ended 9th May (this is a time zone difference, since 23:01 on 8th May, Central European time, was early in the morning 9th May, Soviet time). However, this wasn't the end of the war in Europe - the last major Soviet offensive continued until 11th May, and the Battle of Odžak, between the Ustashe and the Yugoslav partisans continued until 25th May. The last German forces, on the Dutch island of Schiermonnikoog, only surrendered 11th June (a negotiated surrender, with no fighting). This was not the end of the fighting, as partisan resistance against the Soviet Union continued for years longer in the Baltic states and Ukraine.

Official dates for the end of the war against Japan vary. The British (and Australian, Canadian, and Indian), Korean, and Dutch VJ Day is 15th August, the date of the Japanese surrender (Japan time). The US VJ Day is 2nd September, the date of the signing of the surrender document on USS Missouri (Japan time). The Soviet Union and Russia, China, and the Philippines celebrate 3rd September as VJ Day (Japanese forces in the Philippines surrendered 3rd September). Hong Kong, before its return to China, celebrated 30th August, when control was returned to the British. Singapore's VJ Day was 12th September, the date that Japanese forces in Singapore surrendered (it ceased being a public holiday after independence).

There are also three main starting dates for the war against Japan: 7th December, the date of the Japanese attack on Pearl Harbor (local time), 8th December, the date of the Japanese attacks against the Western powers in SE Asia (local time), and 7th July, the date the Second Sino-Japanese War began (in 1937). The Second Sino-Japanese War provides an alternative date for the beginning of WWII; the war certainly became a major part of WWII, and has been seriously proposed as the "correct" start of the war, e.g., at https://www.nationalww2museum.org/war/articles/liberation-china-and-pacific

The beginning of WWI is conventionally 28th July, 1914, when Austria-Hungary declared war on Serbia, a month after the assassination of Archduke Franz Ferdinand on 28th June. Germany declared war on Russia 1st August, invaded Belgium and declared war on France 3rd August, and Britain declared war on Germany 4th August. Thus, the major combatants other than Italy, Turkey, and the USA entered the war. As with WWII, the conventional start was hardly a "world war", and one could reasonably suggest that 1st August would be a better date, since this officially brought the number of major powers involved to 3 (and due to African and Asian colonies, spread the war outside Europe).

The end of the war, 11th November 1918, has little competition - the fighting mostly ended then. German forces in East Africa only learned of the armistice on 14th November, immediately agreeing to a ceasefire and surrendering shortly afterwards, on 23rd November. Many of the combatants in WWI continued to fight, intervening in the Russian Civil War which would continue into mid-1923.