The war diary date is quite clear, 21st was when he was wounded (the 2 men killed that day are buried in the same cemetery). 22nd is when he succumbed to his wounds in the field ambulance and is the date on the Commonwealth War Graves entry and the effects register.
My earlier post was incorrect when I said the battery was in Woesten. It was the brigade headquarters and the wagon lines which were there where the casualties were sustained. The brigade headquarters was to the south east of the village and my guess is that they were occupying the wooded area near the "straat" of Elverdingstraat (you need to zoom out a bit of the map I linked to). Canada Farm Cemetery can also be seen on the image, look for Omloop-Zuid and then left to Elzendammestraat, the cemetery is on the "mm" at the cross roads.
There was no specific action taking place at the time. By coincidence, 21 March was the start of the massive German offensive called Operation Michael but this was taking place more to the south of Ypres. The guns of the battery were "on loan" to another division at the time, involved in what the
diary describes as "normal trench warfare" and supporting small scale raids by the infantry. William's job was away from the firing lines, although not immune to incoming shelling, looking after the many horses that were not away with the guns. 11 Brigade had 4 batteries each with something like 200 horses so they took some looking after!
Do come back if I have skated over it too much. This should at least have given you three places that relate closely to him that you can visit.
If it will help, I can put these locations on one map, let me know.
MaxD