Well...every reference in 6.1.12. is to "Etihad Guest Miles" only.

It makes no sense, anyway, for Tier Miles to expire after 2, 2.5, or 3 years (in the way that Guest Miles do, dependent on status).

To achieve a higher status, you must accumulate the necessary (higher) Tier Miles total within a rolling year. However, to prolong/maintain status, you need to accumulate the relevant (lower) Tier Miles total in the calendar year following upgrade/previous maintenance (The so-called "renewal" period - see the tables at the bottom of each individual page for each of the status levels here).

As such, a Tier Mile becomes irrelevant to achieving higher status after 365 days have passed since it was earned; that same Tier Mile has already become irrelevant as regards maintaining status at the end of the calendar year in which it was earned.