Timeline as far as I know extracted from his official Military Service Records, including a few other related highlights.
3 January 1891 - Born in Old Hill, near Cradley Heath, West Midlands (was Staffordshire)
2 April 1911 - The 1911 UK Census states Harry's occupation as "General Labourer - Tube Works" the same as his father
5 July 1911 - Well before the outbreak of the first world war Harry enlisted as a Regular in the Grenadier Guards 1st Battalion for 3 years service. His Enlistment document states he was 20 yrs and 6 months old with a fresh complextion, grey eyes, brown hair and standing 5 ft 11.5 inches tall. (The height limit at that time for the Guards was 5 ft 10 inches). His occupation stated Iron Moulder. Prior to joining the Guards he had also spent some time in the 7th Btn Worcestershire Regiment (TF) Territorial Force.
20 June 1912 - Promoted from Private (also known as Guardsman) to Lance Corporal. As a 'Grenadier' Harry would have worn the classic red tunic and bearskin as they had special responsibilities to guard both St. James and of course Buckingham Palace. 
15 July 1912 - Extends his service from 3 to 7 years with 'The Grenadiers'
14 February 1914 - Promoted to Corporal

28 June 1914 - Austrian Archduke Franz Ferdinand assassinated at Sarajevo

28 July 1914 - Austria-Hungary declares war on Serbia

1 August 1914 - Germany declares war on Russia

3 August 1914 - Germany declares war on France

4 August 1914 - Britain declares war on Germany
4 October 1914 - Harry and the 1st Battalion are sent off to war, they sail from Southampton to Zeebrugge aboard the S.S. Armenian. He was part of the 7th Division, 20th Brigade which made up the British Expeditionary Force (BEF). This division was formed in September 1914 from units which had been serving in the far flung corners of the Empire and had returned to England following the outbreak of war. 
6 October 1914 - Harry disembarked at Zeebrugge, Belgium and initially the 7th Division was given orders to assist in the defence of Antwerp, but this city had fallen before the division even had the chance to get there. Instead they marched south to Ypres, and took part in The First Battle of Ypres, by the close of November had lost more than 9,700 men since arriving in Flanders. Some battalions were all but wiped out. 

At the end of November because of the ferocity of the fighting during The First Battle of Ypres there were only 4 officers and 200 men of Harrys' 1,000 strong 1st Battalion still standing.
 Harry was part of what came to be known as the original  'Old Contemptibles'.
24 October 1914 - Promoted to Sergeant
27 October 1914 - 1st Battalion moved from billets outside Ypres to a bivouac in Sanctuary Wood
26 December 1914 - Harry sends a postcard from the trenches to his fiancée Olive explaining that he'd met with Germans in no mans land during what is now known as the unique and historic 'Christmas Truce'. He also had the presence of mind to obtain the names and addresses of two German soldiers.
One of the two soldiers, Rudolf Mausolff, is remembered on a war memorial in Bad Pyrmont the town Harry wrote down on the postcard. To see information on a German website regarding the monument click here. The memorial is only 1 mile from Rudolfs' family home at Oesdorfer Straße 41, 31812 Bad Pyrmont, Germany.
16 May 1915 - One of the soldiers he met during the Christmas Truce, Gefreiter (Lance Corporal) Rudolf Mausolff, is killed in action. The exact location and details are unknown. 

9 February 1916 - Harry marries Olive Doretta Cook in a Dudley Register Office. Olive was from Boddinick near Fowey, Cornwall but working as a domestic servant in London where she met Harry. Her address at the time is stated as 11 Westmoreland Street, Pimlico, SW1 according to Harry's Military History Sheet.
24 March 1916 - The 1st Battalion had two days rest in Poperinge, Flanders
25 March 1916 - Harry is transferred from the 1st to the 5th Grenadier Guards (Reserve) Battalion
29 March 1916 - After 1 year and 176 days at war, Harry returns to England. He spent some time recovering from injury at a temporary Hospital based in Piddletrenthide, Dorset as shown in some of his photographs. It is not known what his injury was. 

25 September 1916 - The postmark on a letter to Harry dated 25 Sep 1916 indicates that he was still recovering at The Manor House on the B3143, Piddletrenthide, Dorset DT2. Once he recovered he would have returned to Caterham in Surrey which was the Grenadier Guards depot and training camp known as "Little Sparta".

16 May 1917 - Appointed Acting Company Quartermaster Sergeant (CQMS) while stationed at 'Little Sparta'
18 January 1918 - Harry's son William Samuel Harry Hackett is born in London
6 April 1918 - Relinquished his rank of Quartermaster Sergeant and reverted to Sergeant. Transferred to active duty as a reinforcement to the 4th Battalion who were involved in some brutal action as part of several major German assaults across the Western Front.
6 April 1918 - Returns to active duty in France as part of the 4th Battallion Grenadier Guards

7 April 1918 - The German 6th Army launch the second part of General Ludendorff's strategic plan named Operation Georgette. The objective was to take back Ypres it was known as the Battle of the Lys.

With the situation turning desperate, General Haig issued his now famous 'BACKS TO THE WALL' Order of the Day to the British Army.

"There is no other course open to us but to fight it out ! Every position must be held to the last man: there must be no retirement. With our backs to the wall and believing in the justice of our cause, each one of us must fight on to the end".

11 - 14 April 1918 - The 4th Battallion was involved in some of the fiercest fighting of the war in which many soldiers carried out Haigs order to the letter. One such account is that of Captain T Pryce of No 2 Company who was posthumously awarded the Victoria Cross (VC) for his actions. CLICK HERE as it's well worth read the full original 4th Btn account here as it shows the ferocity of the fighting. They even end up fighting with sticks and fists. 

18 April 1918 -  Official war diary accounts state that the 4th Btn Grenadier Guards had been so badly mauled and has lost so many men due to the severe fighting that they and the 2nd Btn Coldstream Guards were merged to make up a composite battalion - a thing never before done in the history of the regiments. This composite battalion moved into billets at Le-Tir-Anglais and on the 22nd they took over the front line from the King's Own Yorkshire Light Infantry where they suffered under a huge bombardment of gas and high explosive shells. This is most likely where Harry fell. It's recorded that Lieutenant R Rolfe was killed here too, he now lies next to Harry in Hazebrouck cemetery along with two other 'brothers-in-arms', all four Grenadier Guards died on the same day. 

La Motte au Bois is a couple of miles south of Hazebrouck and on the road between these 2 places there is a tiny hamlet (only 2 or 3 houses) with a sign saying "Le Tir Anglais", which translates into The English Shooting apparently.

22 April 1918 - Only 16 days after returning to France, Sergeant Harry Hackett died of wounds sustained during the Battle of The Lys. 

Harrys' record states that he died in the 94th Field Ambulance. He had spent a total of 6 years and 293 days in The Guards. 
11 November 1918 - Armistice Day. At 11:00am the Great War ended. In total 15 million people had died.

                       Mobilized            Dead         Wounded     Missing/PoW
Germany               11,000,000       1,773,700       4,216,058       1,152,800
Russia                12,000,000       1,700,000       4,950,000       2,500,000
France                 8,410,000       1,375,800       4,266,000         537,000
Austria-Hungary        7,800,000       1,200,000       3,620,000       2,200,000
Great Britain          8,904,467         908,371       2,090,212         191,652

Today - Harry rests in Cinq Rues Cemetery, Hazebrouck, Flanders, Northern France.

Service Record - (Note 'Home' may refer to either serving in England or recovering from injury)

Location                From           To         Years     Days
Home                03/07/11     03/10/14             3       93
Western Front       04/04/14     28/03/16             1      176
Home                29/03/16     05/04/18             2        8
Western Front       06/04/18     22/04/18             -       16
Total                                                 6      293

All of the information in this site is stuff I've picked up myself from various sources. It's purely for the benefit of my family and those who have any kind of interest .
Andy Rudall (c) Oct 2011