Question: When did Wilfred Owen write "The Letter"?

Context articles: Apologia Pro Poemate Meo
A New Heaven
The Letter (poem)
Dulce et Decorum est
Harold Owen
Wilfred Owen
Wilfred Owen: A Remembrance Tale
Bullets and Daffodils
Dominic Hibberd
Futility (poem)

"Apologia Pro Poemate Meo" is a poem by Wilfred Owen.
 It deals with the atrocities of World War I.
 The title means "in defence of my poetry" and is often viewed as a rebuttal to a remark in Robert Graves' letter "for God's sake cheer up and write more optimistically - the war's not ended yet but a poet should have a spirit above wars."
"A New Heaven" is a sonnet by Wilfred Owen, written in England before Owen had seen active service in the trenches of France, probably in September 1916.
 Some MS drafts bear differing dedications ("To — on active service" or "To a comrade in Flanders").
 The poem was probably written in Milford Camp, Surrey, which was a part of Witley Camp.
"The Letter" is a poem by Wilfred Owen.
 It deals with the atrocities of World War I.
"Dulce et Decorum est" (read , on WikiSource) is a poem written by Wilfred Owen during World War I, and published posthumously in 1920.
 The Latin title is taken from the Roman poet Horace and means "it is sweet and honorable...", followed by "pro patria mori", which means "to die for one's country".
 One of Owen's most renowned works, the poem is known for its horrific imagery and condemnation of war.
 It was drafted at Craiglockhart in the first half of October 1917 and later revised, probably at Scarborough but possibly Ripon, between January and March 1918.
 The earliest surviving manuscript is dated 8 October 1917 and addressed to his mother, Susan Owen, with the message "Here is a gas poem done yesterday (which is not private, but not final)."
Harold Owen (5 September 1897 - 26 November 1971) was the younger brother and biographer of the English poet and soldier, Wilfred Owen.
 He was born at the home of his paternal grandparents in Canon Street, Shrewsbury, Shropshire, where his parents and older siblings then lodged before his father moved on promotion to a station master's post at Birkenhead in 1898.
Wilfred Edward Salter Owen, MC (18 March 1893 – 4 November 1918) was an English poet and soldier, one of the leading poets of the First World War.
 His war poetry on the horrors of trenches and gas warfare was heavily influenced by his mentor Siegfried Sassoon, and stood in stark contrast both to the public perception of war at the time and to the confidently patriotic verse written by earlier war poets such as Rupert Brooke.
 Among his best-known works – most of which were published posthumously – are "Dulce et Decorum est", "Insensibility", "Anthem for Doomed Youth", "Futility", "Exposure" and "Strange Meeting".
Wilfred Owen: A Remembrance Tale was a 1-hour 2007 BBC documentary on the life of the First World War poet Wilfred Owen.
 It was presented by Jeremy Paxman and starred Samuel Barnett as Owen and Deborah Findlay as his mother Susan.
 It premiered on BBC One on Remembrance Sunday 2007.
Bullets and Daffodils is a musical about the life of the war poet Wilfred Owen, created by musician and composer Dean Johnson and directed by Dean Sullivan.
 The musical is based on Owen's poems set to music by Johnson, with the addition of new songs written by Johnson to help narrate the story of Owen's life.
John William Dominic Hibberd FRSL (3 November 1941 – 12 August 2012) was an English freelance author, academic and broadcaster, best known for his biographies of the poets Wilfred Owen and Harold Monro and his collections (edited with John Onions) of First World War poetry.
 He was an Honorary Vice-President of the Wilfred Owen Association and an Honorary Fellow of the War Poets Association.
"Futility" is a poem written by Wilfred Owen, one of the most renowned poets of World War I.
 The poem was written in May 1918 and published as no. 153 in "The Complete Poems and Fragments".
 The poem is well known for its departure from Owen's famous style of including disturbing and graphic images in his work; the poem instead having a more soothing, somewhat light-hearted feel to it in comparison.
 A previous secretary of the Wilfred Owen Association argues that the bitterness in Owen's other poems "gives place to the pity that characterises his finest work".
 Futility details an event where a group of soldiers attempt to revive an unconscious soldier by moving him into the warm sunlight on a snowy meadow.
 However, the "kind old sun" cannot help the soldier - he has died.