ℤ is a microblog for science: the same machinery as 𝕐, on one subject instead of on everything. It has the same composer (with a 𝕐/ℤ switch in its bar), the same month calendar, the same reverse-chronological month pages, the same {{startthread}} markers and the same isolated-entry viewer.
The two are kept apart by two conventions, and by nothing else: a ℤ month page is titled Z/Month (YYYY) rather than Month (YYYY), and a ℤ entry's permalink anchor begins Z- rather than Y-. Every gadget reads the branch off the page it is on, so there is one copy of the code, not two.
The News page and the year diary (2026) are shared: a ℤ post can be filed to either with the composer's checkboxes, exactly as a 𝕐 post can.