The spell checker properly ignores plurals but is does not ignore possessives. Using the word "day" as an example "day" and "days" are both OK but "day's" will be flagged as a spelling error. Any word that ends in 's should spell check OK if the base word spell checks.
In the meantime, the workaround is to use an accent aigu (-?) instead of a straight apostrophe ('). The spell checker doesn't recognize a genitive s that's separated with the latter...