Biff is a command line datetime Swiss army knife

I'm the author of Biff. I just wanted to share a really cool example of something that Biff can do that I _think_ is kinda hard to do otherwise. (And also, I want to make an assertion about it and I hope this will lead to me being wrong and learning something new.)

The use case is: "I want to see a list of all files in a repository, sorted in ascending order of when it was most recently changed according to source control. I also want to highlight the time with color, make it be in local time and format it in my own bespoke way using strftime." Here's the full command (run from the root of https://github.com/BurntSushi/ripgrep):

    $ git ls-files |
        biff tag exec git log -n1 --format='%aI' |
        biff time in system |
        biff time sort |
        biff time fmt -f '%a %Y-%m-%d %H:%M:%S' |
        biff untag -f '{tag}|t{data}'
    ...
    Thu 2025-10-30 13:30:14 crates/ignore/Cargo.toml
    Sat 2025-11-29 14:11:38 crates/core/flags/lowargs.rs
    Wed 2025-12-17 11:38:12 tests/misc.rs
    Wed 2025-12-17 11:38:12 tests/util.rs
    Thu 2026-02-12 20:39:46 crates/ignore/src/default_types.rs
    Fri 2026-02-20 16:06:29 crates/core/flags/config.rs
    Fri 2026-02-27 11:25:19 GUIDE.md
    Fri 2026-02-27 11:25:19 crates/core/flags/defs.rs
    Mon 2026-05-25 23:56:53 CONTRIBUTING.md
    Tue 2026-05-26 08:32:43 AI_POLICY.md

Or even ask for a specific time window:

    $ git ls-files |
        biff tag exec git log -n1 --format='%aI' |
        biff time in system |
        biff time cmp ge 2026-01-01 |
        biff time cmp lt 2026-04-01 |
        biff time sort |
        biff time fmt -f '%a %Y-%m-%d %H:%M:%S' |
        biff untag -f '{tag}|t{data}'
    Thu 2026-02-12 20:39:46 crates/ignore/src/default_types.rs
    Fri 2026-02-20 16:06:29 crates/core/flags/config.rs
    Fri 2026-02-27 11:25:19 GUIDE.md
    Fri 2026-02-27 11:25:19 crates/core/flags/defs.rs

If you run this on a big repository, it will take quite a lot of time because `git log -n1` takes a long time. I think this is the fastest way to get the most recent commit time on a single file? (That's the assertion that I hope someone can correct me on!) In any case, `biff tag exec` is using parallelism under the hood to make this even faster.

No, Biff informs the system whether you want to be notified when mail arrives during the current terminal session.

I am a happy user of dateutils [0], but I will try out Biff and see which one is more ergonomic.

[0]: https://www.fresse.org/dateutils/

Same dude that made jiff. Love that library, so I'm assuming biff is built on top of jiff.

I remember when biff was what we ran in a CSH to be informed of new email. I don’t remember if this was a local UCB tool or if it was part of BSD.

% biff

2026 M05 28, Thu 17:27:46

Ahh, the month of M05

The thing Biff gets right that gnu `date` and most stdlib datetime APIs get wrong: it treats "civil time" and "absolute instants" as different types. You cannot answer "what's 30 days from 2024-03-08 in America/New_York" without picking a side — DST means that's either 29d23h or 30d0h of elapsed time, and most APIs silently pick one without telling you.

Jiff (the underlying Rust crate) gets this from Temporal in TC39, which is the first time JS standards have led anything datetime-shaped. Hopefully the rest of the ecosystem catches up — Python's `zoneinfo` only landed in 3.9 and `datetime.timezone` still has sharp edges.