Addendum
2023-12-01 - Karl Seguin’s zig utils also have some datetime functionality: GitHub - karlseguin/zul: zig utility library
2023-12-10 - @geemili revived the Proposal: DateTime in std.time.
2024-01-08 - PR closed https://github.com/ziglang/zig/pull/18272
2024-04-27 - PR closed: `std`: add generic `date.Date`, `time.Time`, and `date_time.DateTime` by clickingbuttons · Pull Request #19549 · ziglang/zig · GitHub (now in a separate repo: GitHub - clickingbuttons/datetime: Generic Date, Time, and DateTime library.)