Mlife tier status cutoff is 12/31 or 1/31/2025

Yes, they operate the same as CET now.

Earning year is 1/1 - 12/31

Tier Credits reset to zero on 1/1.

Tier Benefits/Status last until 1/31, so if you're Gold for example and you have a stay in January, you'd get automatic resort fee waiver even if you didn't re-earn Gold. Your tier status won't reset until 2/1.

One note if he is going to go for a daily tier accelerator bonus:

The accelerator promo is currently scheduled to end on 12/31 (it may be extended again, don't know yet).

Bonus TCs from the accelerator aren't posted until the day after.

So if you're counting on bonus TCs to reach a tier threshold, you have to trigger the accelerator on 12/30 in order to have the bonus TCs post on 12/31 and count for 2024.

If you reach an accelerator threshold on 12/31, the bonus TCs won't post until 1/1 and they'll count for 2025.