The t’s and c’s say in 8.3:
8.3. On receipt of a satisfactory application form British Airways shall at its discretion issue to the Nominee a discretionary Silver/Gold Partner Card which will be valid for a minimum of 12 months from the date of issue. For Nominees who are existing Executive Club Members, the discretionary British Airways Silver/Gold Card period shall begin from the date of acceptance by British Airways of the Nominee and last until the Nominee’s Membership Year End date and for the 12 month period thereafter. For Nominees who are not existing Executive Club Members at time of nomination Silver/Gold Cards will be valid for a period of 12 months from the date of acceptance by British Airways of the Nominee.
Membership Year End means the expiry date on the Membership Card;
GGL are now enforcing:
the period to the expiry date of the then current card at the point of nomination acceptance, plus 1 year; as distinct from
the period to the expiry of the then current tier point collection year at the time of nomination, plus 1 card year.
So it’s important to nominate after the expiry of the current card, to get nearly 2 years, otherwise you may get just over 1 year, depending on dates.