The medical record mentioned above is a transcription:
Name -- William Henry Gee
Birth Year -- 1898
Age -- 19
Rank -- Gunner
Service No -- 82616
Regiment or unit -- Royal Garrison Artillery
Sub0unit -- 75 Anti Aircraft Battery attached to RGS K Anti Aircraft Battery
Hospital -- Red Cross Hospital, East Harnham, Salisbury
Length of service 15 months
Service 'in the field' 13 months
Arrived back in UK from France -- Hospital Ship Warilda
Disease or injury -- Trench Nephritis (kidney infection)
Transfer -- to Rushmore Auxiliary Hospital, Salisbury on the 5th October 1917
He is shown as being in the Army for 15 months -- this would give an estimated enlistment of July 1916 and in a theatre of war (France) estimated at September 1916