It looks like the CWGC is the correct date. I checked out other 11th Bn casualties on both dates using Geoff's Search Engine and the quote below from the CWGC Cemetery Details seems to back it up.
"The site of the Menin Gate was chosen because of the hundreds of thousands of men who passed through it on their way to the battlefields. It commemorates those of all Commonwealth nations except New Zealand who died in the Salient, in the case of United Kingdom casualties before 16 August 1917. Those United Kingdom and New Zealand servicemen who died after that date are named on the memorial at Tyne Cot,...."
Phil