I don’t know how those who marked down July 28th, 1991 came to that conclusion, but they were magnificently wrong by a margin of 72 years, 8 months, and 17 days. It could be they were thinking of July 28th, 1914 which was the day the war began, but the fighting most certainly ended on November 11th, 1918. Now as for the Treaty of Versailles, which formally ended the state of war between Germany and the Entente Powers, that was signed on June 28th, 1919. The last of the treaties was signed on July 24, 1923 in Lausanne, ending the state of war between the Ottomans and their adversaries.

Edit: on mobile, accidentally hit send too early.