Ponies are stamped by the year that mould was first used. It has nothing to do with their release year unless they are the first pony in that pose.
Of course, in case of ponies like Up Up and Away, it also depends on the region. She's one of the first in that pose, her pose is dated 1987 and her US release is 1987. In Europe she's released in 1988, but she still has 1987 on her hooves.
So yeah. Date is the date of the mould, not the date of the pony release, unless they are the first to use that pose. Actual release date could be much later, and depends on the region of release.