It should be after 1950, to the beginning of the 20th century. It can be known that in 2006, he selected the poem for WPA, which is the website of the War Poets Association.
Vernon Scannell (b. 1922) has been publishing his poetry since the 1950s, but seems to be less well-known than he deserves, despite being the recipient of the Heinemann Award for Literature and the Cholmondely Poetry Prize .
In addition, http://www.warpoets.org/conflicts/ww2/scannell/
The specific year cannot be determined.
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Have a look here: http://www.studentcentral.co.uk/AS_and_A_Level/English_Literature/Poetry/Post-1770/War_Poetry/setting_of_the_poem_contributes_greatly_L133814.html