Each time you are promoted (whether from Blue to Silver, or from Silver to Gold), your Tier Points are reset to zero, and a new membership year is started (the dates delineating your previous membership year become irrelevant to EC upon promotion).

In other words, to reach Gold from Blue, you must first earn 400 points to achieve Silver, and then another 800 points to achieve Gold. BAEC considers each of these to be a separate "membership year" whether it lasts 12 months, or in an extreme case only one month!

Once you reach Gold, your points will reset again (and a new membership year will start). After this promotion to Gold, your membership year will indeed last 12 (or a little more) months, as there are no more "promotions" that could cause the tier points to reset and another year to be started.

Good luck, and enjoy your new Silver Card!