Per Mac Office Help:
If some text is marked with the Do not check spelling or grammar format, Word skips it during a spelling and grammar check. For example, Word automatically marks fields such as DATE and TIME with the Do not check spelling or grammar format so the spelling and grammar checker won't question them. To locate such text or fields and remove the Do not check spelling or grammar format:
1. On the Edit menu, click Replace.
2. In the Find what box, delete any text.
3. Click the arrow show column browser in the lower left corner of the Find and Replace dialog box, and then click Format.
4. Click Language, and then select the Do not check spelling or grammar check box.
5. In the Replace with box, delete any text, and then click Format.
6. Click Language, and then clear the Do not check spelling or grammar check box.
7. Click Find Next, and then click Replace