bttf

A command line tool for datetime arithmetic, parsing, formatting and more.

Dual-licensed under MIT or the UNLICENSE.

Documentation

The user guide should be your first stop for understanding the high level concepts that bttf deals with. Otherwise, consult bttf --help or bttf <sub-command> --help for more specific details.

Alternatively, there is a comparison between other similar tools that might give you a quick sense of what bttf is like.

Brief Examples

Print the current time:

$ bttf
Sat, May 10, 2025, 8:02:04 AM EDT

Tip

If you get output like 2025 M05 10, Mon 08:02:04 instead, that's because you likely don't have locale support support configured. That requires setting BTTF_LOCALE and using a GitHub release binary or building bttf with the locale feature enabled.

Print the current time in a format of your choosing:

$ bttf time fmt -f rfc3339 now
2025-05-10T08:08:30.101066734-04:00

$ bttf time fmt -f rfc9557 now
2025-05-10T08:08:33.420946447-04:00[America/New_York]

$ bttf time fmt -f '%Y-%m-%d %H:%M:%S %Z' now
2025-05-10 08:08:48 EDT

Print multiple relative times in one command:

$ bttf time fmt -f '%c' now -1d 'next sat' 'last monday' '9pm last mon'
Sat, May 10, 2025, 10:44:39 AM EDT
Fri, May 9, 2025, 10:44:39 AM EDT
Sat, May 17, 2025, 10:44:39 AM EDT
Mon, May 5, 2025, 10:44:39 AM EDT
Mon, May 5, 2025, 9:00:00 PM EDT

Print the current time in another time zone, and round it the nearest 15 minute increment:

$ bttf time in Asia/Bangkok now | bttf time round -i 15 -s minute
2025-05-10T19:15:00+07:00[Asia/Bangkok]

Add a duration to the current time:

$ bttf time add -1w now
2025-05-03T10:34:30.819577918-04:00[America/New_York]

$ bttf time add '1 week, 12 hours ago' now
2025-05-02T22:34:44.114109514-04:00[America/New_York]

$ bttf time add 6mo now
2025-11-10T10:34:49.023321635-05:00[America/New_York]

Find the duration since a date in the past and round it to the desired precision:

$ bttf span since 2025-01-20T12:00
2636h 1m 21s 324ms 691µs 216ns

$ bttf span since 2025-01-20T12:00 -l year
3mo 20d 21h 1m 25s 171ms 886µs 534ns

$ bttf span since 2025-01-20T12:00 | bttf span round -l year -s day
3mo 18d

$ bttf span since 2025-01-20T12:00 | bttf span round -l day -s day
110d

Find timestamps in a log file and reformat them into your local time in place:

$ head -n3 /tmp/access.log
2025-04-30T05:25:14Z    INFO    http.log.access.log0    handled request
2025-04-30T05:25:17Z    INFO    http.log.access.log0    handled request
2025-04-30T05:25:18Z    INFO    http.log.access.log0    handled request

$ bttf tag lines /tmp/access.log | bttf time in system | bttf time fmt -f '%c' | head -n3 | bttf untag -s
Wed, Apr 30, 2025, 1:25:14 AM EDT       INFO    http.log.access.log0    handled request
Wed, Apr 30, 2025, 1:25:17 AM EDT       INFO    http.log.access.log0    handled request
Wed, Apr 30, 2025, 1:25:18 AM EDT       INFO    http.log.access.log0    handled request

Generate a sequence of the next 5 days that are Monday, Wednesday or Friday at a specific time, and then format them in your locale:

$ bttf time seq day today -c5 -H 9 -w mon,wed,fri | bttf time fmt -f '%c'
Mon, May 12, 2025, 9:00:00 AM EDT
Wed, May 14, 2025, 9:00:00 AM EDT
Fri, May 16, 2025, 9:00:00 AM EDT
Mon, May 19, 2025, 9:00:00 AM EDT
Wed, May 21, 2025, 9:00:00 AM EDT

Print every day remaining in the current month:

$ bttf time seq daily --until $(bttf time end-of month now) today
2025-05-10T00:00:00-04:00[America/New_York]
2025-05-11T00:00:00-04:00[America/New_York]
2025-05-12T00:00:00-04:00[America/New_York]
2025-05-13T00:00:00-04:00[America/New_York]
[.. snip ..]

Find the last weekday in each of the next 12 months and print them in a succinct format:

$ bttf time seq -c12 monthly -w mon,tue,wed,thu,fri --set-position -1 | bttf time fmt -f '%a, %Y-%m-%d'
Fri, 2025-05-30
Mon, 2025-06-30
Thu, 2025-07-31
Fri, 2025-08-29
Tue, 2025-09-30
Fri, 2025-10-31
Fri, 2025-11-28
Wed, 2025-12-31
Fri, 2026-01-30
Fri, 2026-02-27
Tue, 2026-03-31
Thu, 2026-04-30

Or print the second Tuesday of each month until the end of the year:

$ bttf time seq monthly -w 2-tue --until $(bttf time end-of year now) | bttf time fmt -f '%a, %F'
Tue, 2025-05-13
Tue, 2025-06-10
Tue, 2025-07-08
Tue, 2025-08-12
Tue, 2025-09-09
Tue, 2025-10-14
Tue, 2025-11-11
Tue, 2025-12-09

Finally, this command will get the last commit date on each file in a git repository, sort them in ascending order, format the datetime to a fixed-width format and then print the data in a tabular format:

$ git ls-files \
    | bttf tag exec git log -n1 --format='%aI' \
    | bttf time sort \
    | bttf time fmt -f '%Y-%m-%d %H:%M:%S %z' \
    | bttf untag -f '{tag}\t{data}'
[.. snip ..]
2025-05-05 21:54:09 -0400       src/tz/timezone.rs
2025-05-05 21:54:09 -0400       src/tz/tzif.rs
2025-05-05 22:06:38 -0400       Cargo.toml
2025-05-05 22:06:38 -0400       crates/jiff-static/Cargo.toml
2025-05-07 18:55:23 -0400       scripts/test-various-feature-combos
2025-05-07 18:55:23 -0400       src/error.rs
2025-05-08 08:38:22 -0400       src/tz/system/mod.rs
2025-05-08 16:52:55 -0400       crates/jiff-icu/Cargo.toml
2025-05-08 16:52:55 -0400       crates/jiff-icu/src/lib.rs

To see more examples, check out the user guide or the comparison between bttf and other datetime command line tools.

Installation

The binary name for bttf is bttf. It is also on crates.io under the name bttf.

Archives of precompiled binaries for bttf are available for Windows, macOS and Linux. Linux and Windows binaries are static executables.

Alternatively, if you're a Rust programmer, bttf can be installed with cargo. Note that the binary may be bigger than expected because it contains debug symbols. This is intentional. To remove debug symbols and therefore reduce the file size, run strip on the binary.

cargo install bttf

Or, if you want locale support (which is enabled in the binaries distributed on GitHub), then install with the locale feature enabled:

cargo install bttf --features locale

bttf as a library

There is relatively little datetime logic inside of bttf proper. (Except for its RFC 5545 implementation, which may eventually move out to a library.) Most of the datetime logic is instead provided by Jiff. Additionally, localization is provided by ICU4X and integrated with Jiff via jiff-icu.

WARNING

I may ship arbitrary and capricious breaking changes at this point. You have been warned.

Also, no compatibility with date is intended. This is not a drop-in replacement. It is not intended to be. It never will be. And it doesn't give a hoot about POSIX (other than the TZ environment variable). If you need a date compatible program, then go use an implementation of POSIX date. With that said, bttf's bttf time fmt command generally supports a strftime syntax that has a large amount of compatibility with GNU date.

If you have use cases serviced by date that aren't possible with bttf, I'd like to hear about them.

Motivation

I built this tool primarily as a way to expose some of the library functionality offered by Jiff on the command line. I was after a succinct way to format datetimes or do arithmetic. So I built this tool.

date is one of those commands that I use infrequently enough, and its flags and behavior is weird enough, that I constantly have to re-read its manual in order to use it effectively. So perhaps there is room for improvement there.

As I progressed in constructing this tool, I quickly found it somewhat limited by the fact that the only data it could process was datetimes. To make bttf much more versatile, I added a bttf tag command that looks for datetimes in arbitrary data and wraps them up into a JSON lines format. It's unclear to me how broadly useful folks will find this functionality, but other datetime utilities don't seem to have it.

I also wanted to use Jiff in "anger," and in particular, as part of confidently getting it to a 1.0 state. Is its performance acceptable? Are there APIs missing that are needed for real world programs? And so on. For example, because of my development on bttf, I added a way to hook ICU4X localization into Jiff's jiff::fmt::strtime APIs.

Building

bttf is written in Rust, so you'll need to grab a Rust installation in order to compile it.

To build bttf:

git clone https://github.com/BurntSushi/bttf
cd bttf
cargo build --release
./target/release/bttf --version

Additionally, optional locale support can be built with bttf by enabling the locale feature:

cargo build --release --features locale

bttf can be built with the musl target on Linux by first installing the musl library on your system (consult your friendly neighborhood package manager). Then you just need to add musl support to your Rust toolchain and rebuild bttf, which yields a fully static executable:

rustup target add x86_64-unknown-linux-musl
cargo build --release --target x86_64-unknown-linux-musl

Applying the --features locale flag from above should also work.

Running tests

To run both unit tests and integration tests, use:

cargo test

from the repository root. If you're hacking on bttf and need to change or add tests, bttf makes heavy use of cargo insta for snapshot testing. For example, to run tests with Insta, use:

cargo insta test

And if there are any snapshots to review, you can review them via:

cargo insta review