It should not surprise us that historians cannot agree on when WWI started.
This day, July 27, 1914 saw deaths in Mitrovitza, Serbia as Austria invaded a day prior to its formal declaration of war. Francis Joseph Reynolds, II The Story of the Great War: History of the European War 291 (1916):
The first great campaign on the southeastern battle grounds of the Great War began on July 27, 1914, when the Austrian troops undertook their first invasion of Serbia. They crossed the Serbian border at Mitrovitza, about fifty miles northwest of Belgrade, driving the Serbians before them. The first real hostilities of the war opened with the bombardment of Belgrade by the Austrians on July 29, 1914 - six days before the beginning of the campaigns on the western battle fields.
In 1920 Francis William Rolt-Wheeler, wrote also of the first shots. The Boys’ Book of the World War, at 51:
On July 27, although war had not yet been declared, the Austrians invaded Serbia, crossing the border at Mitrovitza on the Save. With the foe actually over their borders, the Serbians took a decisive step. They blew Up the great International Bridge across the Danube, at Belgrade, and, simultaneously, shots were exchanged between the troops on the Austrian and the Serbian banks of the river. Though Austria did not declare war until the following day, these shots at Belgrade, on July 27, 1914, were regarded as the opening fire of the Great War.
On July 28, 1914 Austria-Hungary, alleging that Serbia had not answered its demands in a “satisfactory manner,” declared war at 11:10 a.m. in a telegram to Serbian Prime Minister Nikola Pasic.
The next day, on July 29, 1914, Austro-Hungarian navy ships began bombarding Serbia’s capital, Belgrade.