MR has never counted years at a status level, just years as a MR member (until dropping the requirement about ten years ago). Still, if one had to guess, it would be the same as SPG, namely counting the year at the time it's earned as this would fit with the way that MR extends benefits of the new tier as soon as it's been earned (with points posted and status updated) rather than forcing the elite to wait for the start of the new program year.

However, PP seems to be an exception to this, with PP status being evaluated around the start of the new calendar year and invitations being done around February. AFAIK it's very rare for someone to be invited into PP at any other time since the 2-3% rule is done in a batch. Still anyone getting PP must already be Plat (in fact for at least several years) and there's no need to count years as PP separately from Plat years.