Q. While The Chicago Manual of Style still supports a no-hyphen version of “up to date” when not before a noun, Merriam-Webster appears to support using hyphens in all cases. I am not sure which to advise my clients to use.
A. When Merriam-Webster and CMOS disagree relative to hyphenation, follow CMOS. Because CMOS says specifically that “up to date” remains open after a noun—an up-to-date solution but his equipment was up to date (see the hyphenation guide at CMOS 7.96, under “phrases, adjectival”)—that’s what you should advise.
Keep in mind that Chicago favors a spare hyphenation style. What this often means in practice is that any compound adjective that follows the noun it modifies (i.e., instead of directly preceding it) can usually be left open. There are only a few adjective phrases that retain hyphens in any position—among them all-consuming and high-spirited—as described in CMOS 7.92.
But those are exceptions. In general, documents that include relatively few hyphens in compound adjectives after a noun will be the ones that are the most up to date as far as Chicago style is concerned (up to and including the 18th edition).