#parse #string #duration #time #fundu


Configurable, precise and fast rust string parser to a Duration

12 releases (1 stable)

new 1.0.0 May 30, 2023
0.5.1 May 1, 2023
0.5.0 Mar 29, 2023
0.4.3 Mar 21, 2023
0.1.0 Feb 1, 2023

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MIT license


Configurable, precise and fast rust string parser to a Duration

Table of Contents


fundu provides a flexible and fast parser to convert rust strings into a Duration. fundu parses into its own Duration but provides methods to convert into std::time::Duration, chrono::Duration and time::Duration. Some examples for valid input strings with the standard feature:

  • "1.41"
  • "42"
  • "2e-8", "2e+8" (or likewise "2.0e8")
  • ".5" or likewise "0.5"
  • "3." or likewise "3.0"
  • "inf", "+inf", "infinity", "+infinity"
  • "1w" (1 week) or likewise "7d", "168h", "10080m", "604800s", ...

For examples of the custom feature see Customization section. Summary of features provided by this crate:

  • Precision: There are no floating point calculations and the input is precisely parsed as it is. So, what you put in you is what you get out within the range of a Duration. (See also Comparison)
  • Performance: The parser is blazingly fast (Benchmarks)
  • Customization: TimeUnits, the number format and other aspects are easily configurable (Customization)
  • Sound limits: The duration saturates at Duration::MAX if the input number was larger than that maximum or if the input string was positive infinity.
  • Negative Durations: The parser can be configured to parse negative durations. Fundu's Duration can represent negative durations but also implements TryFrom for chrono::Duration and time::Duration if the corresponding feature is activated.
  • Error handling: The error messages try to be informative on their own but can also be easily adjusted (See also Examples)

fundu aims for good performance and being a lightweight crate. It is purely built on top of the rust stdlib, and there are no additional dependencies required in the standard configuration. The accepted number format is per default the scientific floating point format and compatible with f64::from_str. However, the number format and other aspects can be customized up to formats like systemd time spans or gnu relative times. See also the examples Examples section and the examples folder. For a direct comparison of fundu vs the rust native methods Duration::(try_)from_secs_f64 see Comparison.

For further details see the Documentation!


Add this to Cargo.toml for fundu with the standard feature.

fundu = "1.0.0"

fundu is split into two main features, standard (providing DurationParser and parse_duration) and custom (providing the CustomDurationParser). The first is described here in in detail, the latter adds fully customizable identifiers for time units. Most of the time only one of the parsers is needed. To include only the CustomDurationParser add the following to Cargo.toml:

fundu = { version = "1.0.0", default-features = false, features = ["custom"] }

Activating the chrono or time feature provides a TryFrom implementation for chrono::Duration or time::Duration.

Activating the serde feature allows some structs and enums to be serialized or deserialized with serde


If only the default configuration is required once, the parse_duration method can be used. Note that parse_duration returns a std::time::Duration in contrast to the parse method of the other parsers which return a fundu::Duration.

use std::time::Duration;

use fundu::parse_duration;

let input = "1.0e2s";
assert_eq!(parse_duration(input).unwrap(), Duration::new(100, 0));

When a customization of the accepted TimeUnits is required, then DurationParser::with_time_units can be used.

use fundu::{Duration, DurationParser};

let input = "3m";
    Duration::positive(180, 0)

When no time units are configured, seconds is assumed.

use fundu::{Duration, DurationParser};

let input = "1.0e2";
    Duration::positive(100, 0)

However, the following will return an error because y (Years) is not a default time unit:

use fundu::DurationParser;

let input = "3y";

The parser is reusable and the set of time units is fully customizable

use fundu::TimeUnit::*;
use fundu::{Duration, DurationParser};

let parser = DurationParser::with_time_units(&[NanoSecond, Minute, Hour]);

assert_eq!(parser.parse("9e3ns").unwrap(), Duration::positive(0, 9000));
assert_eq!(parser.parse("10m").unwrap(), Duration::positive(600, 0));
assert_eq!(parser.parse("1.1h").unwrap(), Duration::positive(3960, 0));
assert_eq!(parser.parse("7").unwrap(), Duration::positive(7, 0));

Setting the default time unit (if no time unit is given in the input string) to something different than seconds is also easily possible

use fundu::TimeUnit::*;
use fundu::{Duration, DurationParser};

    Duration::positive(1, 0)

The identifiers for time units can be fully customized with any number of valid utf-8 sequences if the custom feature is activated:

use fundu::TimeUnit::*;
use fundu::{CustomTimeUnit, CustomDurationParser, Duration};

let parser = CustomDurationParser::with_time_units(&[
    CustomTimeUnit::with_default(MilliSecond, &["χιλιοστό του δευτερολέπτου"]),
    CustomTimeUnit::with_default(Second, &["s", "secs"]),
    CustomTimeUnit::with_default(Hour, &[""]),

assert_eq!(parser.parse(".3χιλιοστό του δευτερολέπτου"), Ok(Duration::positive(0, 300_000)));
assert_eq!(parser.parse("1e3secs"), Ok(Duration::positive(1000, 0)));
assert_eq!(parser.parse("1.1⏳"), Ok(Duration::positive(3960, 0)));

The custom feature can be used to customize a lot more. See the documentation of the exported items of the custom feature (like CustomTimeUnit, TimeKeyword) for more information.

Also, fundu tries to give informative error messages

use fundu::DurationParser;

    "Time unit error: No time units allowed but found: 'y' at column 1"

The number format can be easily adjusted to your needs. For example to allow numbers being optional, allow some ascii whitespace between the number and the time unit and restrict the number format to whole numbers, without fractional part and an exponent (Also note that the DurationParserBuilder can build a DurationParser at compile time in const context):

use fundu::TimeUnit::*;
use fundu::{Duration, DurationParser, ParseError};

const PARSER: DurationParser = DurationParser::builder()
    .allow_delimiter(|byte| matches!(byte, b'\t' | b'\n' | b'\r' | b' '))

assert_eq!(PARSER.parse("ns").unwrap(), Duration::positive(0, 1));
    PARSER.parse("1000\t\n\r ns").unwrap(),
    Duration::positive(0, 1000)

    ParseError::Syntax(1, "No fraction allowed".to_string())
    ParseError::Syntax(1, "No exponent allowed".to_string())

It's also possible to parse multiple durations at once with parse_multiple. The different durations can be separated by an optional delimiter (a closure matching a u8) defined with parse_multiple. If the delimiter is not encountered, a number can also indicate a new duration.

use fundu::{Duration, DurationParser};

let parser = DurationParser::builder()
    .parse_multiple(|byte| matches!(byte, b' ' | b'\t'), Some(&["and"]))

    parser.parse("1.5h 2e+2ns"),
    Ok(Duration::positive(5400, 200))
    Ok(Duration::positive(55, 500_000_000))
assert_eq!(parser.parse("1\t1"), Ok(Duration::positive(2, 0)));
    parser.parse("1.   .1"),
    Ok(Duration::positive(1, 100_000_000))
assert_eq!(parser.parse("2h"), Ok(Duration::positive(2 * 60 * 60, 0)));
    parser.parse("300ms20s 5d"),
    Ok(Duration::positive(5 * 60 * 60 * 24 + 20, 300_000_000))
    parser.parse("300.0ms and 5d"),
    Ok(Duration::positive(5 * 60 * 60 * 24, 300_000_000))

See also the examples folder for common recipes and integration with other crates. Run an example with


like the systemd time span parser example

# For some of the examples a help is available. To pass arguments to the example itself separate 
# the arguments for cargo and the example with `--`
$ cargo run --example systemd --features custom --no-default-features -- --help

# To actually run the example execute
$ cargo run --example systemd --features custom --no-default-features '300ms20s 5day'
Original: 300ms20s 5day
      μs: 432020300000
   Human: 5d 20s 300ms

Time units

Second is the default time unit (if not specified otherwise for example with DurationParser::default_unit) which is applied when no time unit was encountered in the input string. The table below gives an overview of the constructor methods and which time units are available. If a custom set of time units is required, DurationParser::with_time_units can be used.

TimeUnit Default identifier Calculation Default time unit
Nanosecond ns 1e-9s
Microsecond Ms 1e-6s
Millisecond ms 1e-3s
Second s SI definition
Minute m 60s
Hour h 60m
Day d 24h
Week w 7d
Month M Year / 12
Year y 365.25d

Note that Months and Years are not included in the default set of time units. The current implementation uses an approximate calculation of Months and Years in seconds and if they are included in the final configuration, the Julian year based calculation is used. (See table above)

With the CustomDurationParser from the custom feature, the identifiers for time units can be fully customized.


Unlike other crates, fundu does not try to establish a standard for time units and their identifiers or a specific number format. A lot of these aspects can be adjusted when initializing or building the parser. Here's an incomplete example for possible customizations of the number format:

use fundu::TimeUnit::*;
use fundu::{Duration, DurationParser, ParseError};

let parser = DurationParser::builder()
    // Use a custom set of time units. For demonstration purposes just NanoSecond
    // Allow some whitespace characters as delimiter between the number and the time unit
    .allow_delimiter(|byte| matches!(byte, b'\t' | b'\n' | b'\r' | b' '))
    // Makes the number optional. If no number was encountered `1` is assumed
    // Disable parsing the fractional part of the number => 1.0 will return an error
    // Disable parsing the exponent => 1e0 will return an error
    // Finally, build a reusable DurationParser

// Some valid input
assert_eq!(parser.parse("ns").unwrap(), Duration::positive(0, 1));
    parser.parse("1000\t\n\r ns").unwrap(),
    Duration::positive(0, 1000)

// Some invalid input
    ParseError::Syntax(1, "No fraction allowed".to_string())
    ParseError::Syntax(1, "No exponent allowed".to_string())

Here's an example for fully-customizable time units which uses the CustomDurationParser from the custom feature:

use fundu::TimeUnit::*;
use fundu::{CustomDurationParser, CustomTimeUnit, Duration, Multiplier, TimeKeyword};

// Let's define a custom time unit `fortnight` which is worth 2 weeks. Note the creation 
// of `CustomTimeUnits` and `TimeKeywords` can be `const` and moved to compile time:
const FORTNIGHT: CustomTimeUnit = CustomTimeUnit::new(
    &["f", "fortnight", "fortnights"],
    Some(Multiplier(2, 0)),

let parser = CustomDurationParser::builder()
        CustomTimeUnit::with_default(Second, &["s", "secs", "seconds"]),
        CustomTimeUnit::with_default(Minute, &["min"]),
        CustomTimeUnit::with_default(Hour, &["ώρα"]),
    // Additionally, define `tomorrow`, a keyword of time which is worth `1 day` in the future.
    // In contrast to a `CustomTimeUnit`, a `TimeKeyword` doesn't accept a number in front of it 
    // in the source string.
    .keyword(TimeKeyword::new(Day, &["tomorrow"], Some(Multiplier(1, 0))))

    Duration::positive(15120, 0)
    Duration::positive(60 * 60 * 24, 0)
    Duration::positive(60 * 60 * 24 * 7 * 2, 0)


To run the benchmarks on your machine, clone the repository

git clone https://github.com/Joining7943/fundu.git
cd fundu

and then run all benchmarks with

cargo bench --all-features

The iai-callgrind (feature = with-iai) and flamegraph (feature = with-flamegraph) benchmarks can only be run on unix. Use the --features option of cargo to run the benchmarks for specific features:

cargo bench --features standard,custom

The above won't run the flamegraph and iai-callgrind benchmarks.

Benchmarks can be further filtered for example with

cargo bench --bench benchmarks_standard
cargo bench --bench benchmarks_standard -- 'parsing speed'
cargo bench --features custom --no-default-features --bench benchmarks_custom

For more infos, see the help with

cargo bench --help # The cargo help for bench
cargo bench --bench benchmarks_standard -- --help # The criterion help

To get a rough idea about the parsing times, here the average parsing speed of some inputs on a comparatively slow machine (Quad core 3000Mhz, 8GB DDR3, Linux)

Input avg parsing time ~ samples / s
1 37.925 ns 26_367_831.245
123456789.123456789 50.473 ns 19_812_573.058
format!("{}.{}e-1022", "1".repeat(1022), "1".repeat(1022)) 371.02 ns 2_695_272.492

For comparison, the precision and additional features of fundu result in a very low performance overhead due to the initial setup of structures, etc., and quickly catches up. Fundu even becomes more performant than the reference function from the stdlib as the input gets larger (the reference function is Duration::from_secs_f64(input.parse().unwrap())):

Input avg parsing time ~ samples / s
1 25.630 ns 39_016_777.214
123456789.123456789 45.007 ns 22_218_765.969
format!("{}.{}e-1022", "1".repeat(1022), "1".repeat(1022)) 1.7457 µs 572_836.111

Comparison fundu vs Duration::from_secs_f64

Here's a short incomplete overview of differences and advantages of fundu over using Duration::from_secs_f64(input.parse().unwrap()) (and Duration::try_from_secs_f64(input.parse().unwrap()))

Input fundu Duration::(try_)from_secs_f64
01271480964981728917.1 Duration::new(1_271_480_964_981_728_917, 100_000_000) Duration::new(1_271_480_964_981_729_024, 0)
1.11111111111e10 Duration::new(11_111_111_111, 100_000_000) Duration::new(11_111_111_111, 100_000_381)
1ns Duration::new(0, 1) cannot parse time units
"1 2e-3 3e-9", 1s2ms3ns can parse multiple durations as one Duration::new(1, 2_000_003) not possible
1000 When changing the default unit to MilliSecond -> Duration::new(1, 0) is always seconds based
1e20 Duration::MAX panics or returns an error due to: can not convert float seconds to Duration: value is either too big or NaN
infinity Duration::MAX panics or returns an error due to: can not convert float seconds to Duration: value is either too big or NaN
-1, -1s, ... can parse negative durations if enabled panics or returns an error

Platform support

Since fundu is purely built on top of the rust stdlib without platform specific code, this library should be compatible with all platforms. Please open an issue if you find any unsupported platforms which rust itself supports.

See also the CI for platforms on which fundu is tested.


MIT license (LICENSE or http://opensource.org/licenses/MIT)


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