Although only a wound to the arm the time to his discharge can be measured in months.
Hospitalised & then convalescence/rehabilitation before being re-assessed/re-classified.
You could be looking as far back as March.
Having read the diary for that period there was a major raid carried out by the 6th. on the German held Petit-Bois salient.
There were a number of casualties including 66 wounded.
There was also a minor raid the following month with a small number of wounded.
Although he could have been wounded on any day as shelling & sniping was a constant danger this is where he was at the time. They were in this sector until early June when they went into Div. reserve.
Petit-Bois is South of Ypres & North of Armentieres.
The map: 28 S.W. (Wytschaete) Ref : N.24.
http://maps.nls.uk/view/101464681Blue represents the British trench system & red German.
As an aside the C.O. of the 6th. makes reference to the "big push in the South" & the detonation of 19 mines.
This was the battle of Messines Ridge where the largest man made explosion occurred.
It was said Lloyd George heard it in Downing St.