IMO that's not the best idea because of this:
https://en.wikipedia.org/wiki/Week#Week_numbering As you can see, the week numbering depends on the area, where you live. So if you feld is "hardcoded" it may be wrong.
TiddlyWiki atm stores the created and modified fields in this format: YYYYMMDDhhmmssms UTC format
So a tiddler created at Vienna at 12:50 2016-07-29 has a created field like this: 20160729105002497
You can see the difference in hh:mm ?? Vienna is UTC+2
So in my opinion, the week needs to be dynamically calculated, based on "created" and "modified". ... BUT this should be part of the core, to make it right and easy to use.