That's because you can use the Companion Pass during the year you earned it, as well as the entire following year. (millionmilesecrets.com)
Elite status becomes effective immediately after you qualify and continues for the rest of the calendar year, the entire following year, and through February of the next year. (upgradedpoints.com)
This means you'll have Mosaic elite status for the remainder of the year in which you earned it as well as the entire following year. (upgradedpoints.com)