The way I've always read it.
My ssea rating expires on 31 June of odd numbered years, 11--13--15--17 etc.
Counting the hours flown from July first 15 to June 31-17 needs to be at least 12 plus 1 hour with an instructor. The hour with the instructor can be anywhere within the same 24 months. Theoretically 2 July 15 would do.
If these hours and instruction criteria are met then the rating is renewed on its rolling anniversary.
If I do not meet the criteria I would need a proficiency test from an examiner, the date of that test would then reset the new anniversary date for renewal.
With the hours reset to zero.
I can then fly from that date legally for 24 months, or I can fly legally the next day, then not at all until the day before the anniversary, take the hour instruction and fly for 12 hours that day and be legal to continue for another 24 months.