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?