date and date-time are both undefined meta tags that are used in a variety of ways related to dates (and times). Even though these are meta tags, there might be some value in categorizing the subject area.
On the other hand, these are specific functions in a number of applications and answers are not generally applicable from one to another. If it is useful to have dedicated tags for these functions, it may be appropriate to make them application-specific (which would require new tags and contribute to tag bloat).
I'll just put this out there for the community to decide the appropriate course.