tai-time

A nanosecond-precision monotonic clock timestamp based on the TAI time standard.

Overview

While Rust's standard library already provides the std::time::Instant monotonic timestamp, its absolute value is opaque. In many scientific and engineering applications such as simulations, GNSS and synchronized systems, monotonic timestamps based on absolute time references are required.

This crate provides a fairly unopinionated timestamp for such applications with a focus on simplicity, adherence to Rust's std::time idioms and interoperability with the std::time::Duration type.

A TaiTime timestamp specifies a TAI point in time. It is represented as a 64-bit signed number of seconds and a positive number of nanoseconds, relative to 1970-01-01 00:00:00 TAI or to any arbitrary epoch. This timestamp format has a number of desirable properties:

• it is computationally efficient for arithmetic operations involving the standard Duration type, which uses a very similar internal representation,
• when a 1970 epoch is chosen (see MonotonicTime):
  • exact conversion to a Unix timestamp is trivial and only requires subtracting from this timestamp the number of leap seconds between TAI and UTC time,
  • it constitutes a strict 96-bit superset of 80-bit PTP IEEE-1588 timestamps, a widely used standard for high-precision time distribution,
  • it is substantially similar (though not strictly identical) to the TAI64N time format,
• with a custom epoch, other monotonic clocks such as the Global Position System clock, the Galileo System Time clock and the BeiDou Time clock can be represented (see GpsTime, GstTime, BdtTime, Tai1958Time and Tai1972Time).

MonotonicTime, an alias for TaiTime with an epoch set at 1970-01-01 00:00:00 TAI, is the recommended timestamp choice when no specific epoch is mandated.

On systems where std is present, TaiClock can generate TAI timestamps based on the monotonic system clock. On platforms that support it (currently, only Linux), the native TAI system clock time can be retrieved with TaiTime::now.

Usage

Add this to your Cargo.toml:

[dependencies]
tai-time = "0.3.3"

Examples

Basic usage:

use tai_time::{GpsTime, MonotonicClock, MonotonicTime};

// A timestamp dated 2009-02-13 23:31:30.987654321 TAI.
// (same value as Unix timestamp for 2009-02-13 23:31:30.987654321 UTC).
let t0 = MonotonicTime::new(1_234_567_890, 987_654_321).unwrap();

// Current TAI time based on the system clock, assuming 37 leap seconds.
let clock = MonotonicClock::init_from_utc(37);
let t1 = clock.now();
println!("Current TAI time: {}", t1);

// Elapsed time between `t0` and `t1`.
let dt = t1.duration_since(t0);
println!("t1 -t0: {}s, {}ns", dt.as_secs(), dt.subsec_nanos());

// Elapsed time since `t1`.
let dt = clock.now().duration_since(t1);
println!("Elapsed: {}s, {}ns", dt.as_secs(), dt.subsec_nanos());

// Print out `t1` as a GPS timestamp.
let gps_t1: GpsTime = t1.to_tai_time().unwrap();
println!("GPS timestamp: {}s, {}ns", gps_t1.as_secs(), gps_t1.subsec_nanos());

Construction from date-time fields and date-time strings:

use tai_time::{MonotonicTime, Tai1958Time};
let t0 = MonotonicTime::try_from_date_time(2222, 11, 11, 12, 34, 56, 789000000).unwrap();
// The `FromStr` implementation accepts date-time stamps with the format:
// [±][Y]...[Y]YYYY-MM-DD hh:mm:ss[.d[d]...[d]]
// or:
// [±][Y]...[Y]YYYY-MM-DD'T'hh:mm:ss[.d[d]...[d]]
assert_eq!("2222-11-11 12:34:56.789".parse(), Ok(t0));

Formatted display as date-time:

use tai_time::MonotonicTime;
let t0 = MonotonicTime::try_from_date_time(1234, 12, 13, 14, 15, 16, 123456000).unwrap();
assert_eq!(
    format!("{}", t0),
    "1234-12-13 14:15:16.123456"
);
assert_eq!(
    format!("{:.0}", t0),
    "1234-12-13 14:15:16"
);
assert_eq!(
    format!("{:.3}", t0),
    "1234-12-13 14:15:16.123"
);
assert_eq!(
    format!("{:.9}", t0),
    "1234-12-13 14:15:16.123456000"
);

Reading TAI time directly from the system clock (Linux-only, requires feature tai_clock):

use tai_time::MonotonicTime;

let now = MonotonicTime::now();

println!("Current TAI time: {}", now);

Design choices and limitations

Leap seconds are never automatically computed during conversion to/from UTC-based timestamps. This is intentional: since leap seconds cannot be predicted far in the future, any attempt to "hide" their existence from user code would lend a false sense of security and, down the line, would make it more difficult to identify failures subsequent to the introduction of new leap seconds.

Features flags

Support for no-std

By default, this crate enables the std feature to access the operating system clock and allow conversion to/from time::SystemTime. It can be made no-std-compatible by specifying default-features = false.

Support for time-related crates

Conversion methods to and from UTC date-time stamps from the chrono crate are available with the chrono feature.

TAI system clock

On Linux only, it is possible to read TAI time from the system clock by activating the tai_clock feature. Be sure to read about possible caveats in TaiTime::now.

Serialization

TaiTime and related error types can be (de)serialized with serde by activating the serde feature.

defmt support

Activating the defmt feature will derive the defmt::Format trait on TaiTime and related error types.

License

This software is licensed under the Apache License, Version 2.0 or the MIT license, at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.