About 10 years ago I searched and found a webarchive to MHTML converter from someone's small site. I recall there was one caveat, something like it didn't include the date in the metadata of the output MHTML.

Sorry I don't have the link on-hand, it'd be on the HDD I pulled from my old Macbook.

Edit: found an alternative script from my notes: https://blog.iefdev.se/2016/01/converting-webarchive-to-mht/

Edit 2: pretty sure this is the original converter, since I noted a caveat that it outputs the timestamp of the conversion not the time the file was originally saved (the script above handles this correctly otoh). https://langui.net/webarchive-to-mht/