I just checked the UK GGL terms and conditions and 8.3 now states:

"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 British Airways Club members, the discretionary British Airways Silver/Gold card period shall begin from the date of acceptance by British Airways and will last until the end of that current month and for the following 12 months. For nominees who aren't existing members of The British Airways Club at the 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."

So according to that it is 12 months plus the balance of the month you get nominated in. Still better than a slap in the face!

Edit - have now been offered this by my friend. I'm going to wait until the 1st Feb to accept because I think that will give me until Feb 28 2027