Date of modification for websites (Chicago style)

Chicago’s style guide supports the use of a date of modification for web material, especially when no publication date is known. Chicago even says that a date of modification is more important/useful than an accessed date.

Rules 14.13, 14.207, and 14.233 recommend including an “updated” date in some cases: “include a publication date or date of revision or modification; if no such date can be determined, include an access date.” It recommends using language such as “updated” or “last modified,” which seem interchangeable.

Screenshots of relevant rules:

As far as I can tell, date of modification is not currently supported via CSL. Does anyone have any recommendations on how this might be added, or how to otherwise address this issue?

I’d just add the modification/update date as the (publication) date. That won’t include the modified on or update on prefix, but given the nature of web publishing, it’s also not always clear what that means.

CSL’s Chicago style will already include accessed dates if and only if there’s no publication date.