Croner

Croner is a fully-featured, lightweight, and efficient Rust library designed for parsing and evaluating cron patterns.

This is the Rust flavor of the popular JavaScript/TypeScript cron parser croner.

Features

• Parse and evaluate cron expressions to calculate upcoming execution times.
• Generates human-readable descriptions of cron patterns.
• Follows POSIX/Vixie-cron standards, while extending it with additional specifiers such as L for the last day and weekday of the month, # for the nth weekday of the month, W for closest weekday to a day of month.
• Evaluate cron expressions across different time zones.
• Supports optional second-, and year granularity
• Supports optional alternative weekday mode to use Quartz-style weekdays instead of POSIX using with_alternative_weekdays
• Allows for flexible combination of DOM and DOW conditions, enabling patterns to match specific days of the week in specific weeks of the month or the closest weekday to a specific day.
• Compatible with chrono and (optionally) chrono-tz.
• Robust error handling.

Crate Features

• serde: Enables serde::Serialize and serde::Deserialize implementations for Cron. This feature is disabled by default.

Why croner instead of cron or saffron?

Croner combines the features of cron and saffron, while following the POSIX/Vixie "standards" for the relevant parts. See this table:

[!NOTE] Tests carried out at 2023-12-02 using cron@0.12.0 and saffron@.0.1.0

Getting Started

Prerequisites

Ensure you have Rust installed on your machine. If not, you can get it from the official Rust website.

Installation

Add croner to your Cargo.toml dependencies:

[dependencies]
croner = "3.0.0" # Adjust the version as necessary

Usage

Here's a quick example to get you started with matching current time, and finding the next occurrence. is_time_matching takes a chrono DateTime:

use croner::Cron;
use chrono::Local;

fn main() {

    // Parse cron expression
    let cron_all = Cron::from_str("18 * * * 5")
      .expect("Couldn't parse cron string");

    // Compare cron pattern with current local time
    let time = Local::now();
    let matches_all = cron_all.is_time_matching(&time).unwrap();

    // Get next match
    let next = cron_all.find_next_occurrence(&time, false).unwrap();

    // Output results
    println!("Description: {}", cron.describe());
    println!("Time is: {}", time);
    println!("Pattern \"{}\" does {} time {}", cron_all.pattern.to_string(), if matches_all { "match" } else { "not match" }, time );
    println!("Pattern \"{}\" will match next time at {}", cron_all.pattern.to_string(), next);

}

To match against a non local timezone, croner supports zoned chrono DateTime's DateTime<Tz>. To use a named time zone, you can utilize the chrono-tz crate.

use croner::Cron;
use chrono::Local;
use chrono_tz::Tz;

fn main() {
    // Parse cron expression
    let cron = Cron::from_str("18 * * * 5")
      .expect("Couldn't parse cron string");

    // Choose a different time zone, for example America/New_York
    let est_timezone: Tz = "America/New_York".parse().expect("Invalid timezone");

    // Find the next occurrence in EST
    let time_est = Local::now().with_timezone(&est_timezone);
    let next_est = cron.find_next_occurrence(&time_est, false).unwrap();

    // Output results for EST
    println!("EST time is: {}", time_est);
    println!(
        "Pattern \"{}\" will match next time at (EST): {}",
        cron.pattern.to_string(),
        next_est
    );
}

This example demonstrates how to calculate the next 5 occurrences of New Year's Eve that fall on a Friday. We'll use a cron expression to match every Friday (FRI) in December (12) and configure dom_and_dow to ensure both day-of-month and day-of-week conditions are met (see configuration for more details).

use chrono::Local;
use croner::parser::CronParser;

fn main() {
    // Parse cron expression for Fridays in December
    let cron = CronParser::builder()
        // Include seconds in pattern
        .seconds(croner::parser::Seconds::Optional)
        // Ensure both day of month and day of week conditions are met
        .dom_and_dow(true)
        .build()
        .parse("0 0 0 31 12 FRI")
        .expect("Couldn't parse cron string");

    let time = Local::now();

    println!("Finding the next 5 New Year's Eves on a Friday:");
    for time in cron.iter_from(time).take(5) {
        println!("{time}");
    }
}

Pattern

The expressions used by Croner are very similar to those of Vixie Cron, but with a few additions and changes as outlined below:

// ┌──────────────── (optional) second (0 - 59)
// │ ┌────────────── minute (0 - 59)
// │ │ ┌──────────── hour (0 - 23)
// │ │ │ ┌────────── day of month (1 - 31)
// │ │ │ │ ┌──────── month (1 - 12, JAN-DEC)
// │ │ │ │ │ ┌────── day of week (0 - 6, SUN-Mon)
// │ │ │ │ │ │       (0 to 6 are Sunday to Saturday; 7 is Sunday, the same as 0)
// │ │ │ │ │ │
// * * * * * *

• Croner expressions have the following additional modifiers:
  • ?: In the Rust version of croner, a questionmark in the day-of-month or day-of-week field behaves just as *. This allow for legacy cron patterns to be used.
  • L: The letter 'L' can be used in the day of the month field to indicate the last day of the month. When used in the day of the week field in conjunction with the # character, it denotes the last specific weekday of the month. For example, 5#L represents the last Friday of the month.
  • #: The # character specifies the "nth" occurrence of a particular day within a month. For example, supplying 5#2 in the day of week field signifies the second Friday of the month. This can be combined with ranges and supports day names. For instance, MON-FRI#2 would match the Monday through Friday of the second week of the month.
  • W: The character 'W' is used to specify the closest weekday to a given day in the day of the month field. For example, 15W will match the closest weekday to the 15th of the month. If the specified day falls on a weekend (Saturday or Sunday), the pattern will match the closest weekday before or after that date. For instance, if the 15th is a Saturday, 15W will match the 14th (Friday), and if the 15th is a Sunday, it will match the 16th (Monday).
  • +: The plus sign can be used as a prefix to the day-of-week field to create a logical AND between the day-of-month and day-of-week fields. By default, the relationship is a logical OR. For example, 0 0 1 * +MON will run only if the 1st of the month is also a Monday.

[!NOTE] Weekday and month names are case-insensitive. Both MON and mon work. When using L in the Day of Week field, it affects all specified weekdays. For example, 5-6#L means the last Friday and Saturday in the month." The # character can be used to specify the "nth" weekday of the month. For example, 5#2 represents the second Friday of the month.

[!NOTE] The W feature is constrained within the given month. The search for the closest weekday will not cross into a previous or subsequent month. For example, if the 1st of the month is a Saturday, 1W will trigger on Monday the 3rd, not the last Friday of the previous month.

It is also possible to use the following "nicknames" as pattern.

Configuration

Croner uses CronParser to parse the cron expression. Invoking Cron::from_str("pattern") is equivalent to CronParser::new().parse("pattern"). You can customise the parser by creating a parser builder using CronParser::builder.

1. Making seconds optional

This option enables the inclusion of seconds in the cron pattern, but it's not mandatory. By using this option, you can create cron patterns that either include or omit the seconds field. This offers greater flexibility, allowing for more precise scheduling without imposing the strict requirement of defining seconds in every pattern.

Example Usage:

use croner::parser::{CronParser, Seconds};

// Configure the parser to allow seconds.
let parser = CronParser::builder().seconds(Seconds::Optional).build();

let cron = parser
    .parse("*/10 * * * * *") // Every 10 seconds
    .expect("Invalid cron pattern");

2. Making seconds optional required

In contrast to Seconds::Optional, the Seconds::Required variant requires the seconds field in every cron pattern. This enforces a high level of precision in task scheduling, ensuring that every pattern explicitly specifies the second at which the task should run.

Example Usage:

use croner::parser::{CronParser, Seconds};

// Configure the parser to require seconds.
let parser = CronParser::builder().seconds(Seconds::Required).build();

let cron = parser
    .parse("5 */2 * * * *") // At 5 seconds past every 2 minutes
    .expect("Invalid cron pattern");

3. dom_and_dow

This method forces the combination of Day of Month (DOM) and Day of Week (DOW) conditions in your cron expressions. It's particularly useful for creating schedules that require specificity in terms of both the day of the month and the day of the week, such as running a task when the first of the month is a Monday, or christmas day is on a friday. Certain libraries use this mode by default.

[!NOTE] While this method provides a way to globally enforce AND logic, the recommended approach is to use the + modifier directly in the cron pattern (e.g., 0 0 1 * +MON). This pattern-level configuration gives you more granular control and is enabled by default.

Example Usage:

use croner::parser::CronParser;

// Configure the parser to enable DOM and DOW.
let parser = CronParser::builder().dom_and_dow(true).build();

let cron = parser
    .parse("0 0 25 * FRI") // When christmas day is on a friday
    .expect("Invalid cron pattern");

4. alternative_weekdays (Quartz mode)

This configuration method switches the weekday mode from the POSIX standard to the Quartz-style, commonly used in Java-based scheduling systems. It's useful for those who are accustomed to Quartz's way of specifying weekdays or for ensuring compatibility with existing Quartz-based schedules.

Example Usage:

use croner::parser::CronParser;

// Configure the parser to use Quartz-style weekday mode.
let parser = CronParser::builder().alternative_weekdays(true).build();

let cron = parser
    .parse("0 0 12 * * 6") // Every Friday (denoted with 6 in Quartz mode) at noon
    .expect("Invalid cron pattern");

Documentation

For detailed usage and API documentation, visit Croner on docs.rs.

Time and Calendar System

Croner uses the chrono crate, which operates on a proleptic Gregorian calendar. This means it treats all dates, historical or future, as if the Gregorian calendar has always been in effect. Consequently, it does not account for historical calendar reforms (e.g., skipped days during the 1582 Gregorian adoption) and will iterate through all dates uniformly. For stability and practical use, Croner supports dates from year 1 AD/CE up to the beginning of year 5000, preventing searches that are too far into the past or future.

Daylight Saving Time (DST) Handling

Croner-rust provides robust and predictable handling of Daylight Saving Time (DST) transitions, aligning with the Open Cron Pattern Specification (OCPS) and Vixie-cron's time-tested behavior. Jobs are categorized based on their time-unit field specifications:

• Fixed-Time Jobs: Jobs with specific numerical values for seconds, minutes, and hours (e.g., 0 30 2 * * *).
• Interval/Wildcard Jobs: Jobs using wildcards (*) or step values (*/N) in their seconds, minutes, or hours fields (e.g., */5 * * * * *).

During DST transitions, Croner-rust behaves as follows:

• DST Gap (Spring Forward): When a scheduled time falls into a non-existent interval (e.g., 2:00 AM jumps to 3:00 AM):
  • Fixed-Time Jobs: Will execute at the first valid second/minute immediately following the gap on the same calendar day.
  • Interval/Wildcard Jobs: Occurrences within the gap are skipped. Subsequent executions resume at the next regularly scheduled interval relative to the new wall clock time.
• DST Overlap (Fall Back): When a scheduled time interval occurs twice (e.g., 2:00 AM falls back to 1:00 AM):
  • Fixed-Time Jobs: Will execute only once, at its first occurrence in wall clock time.
  • Interval/Wildcard Jobs: Will execute for each occurrence that matches its pattern in wall clock time within the duplicated hour.

Development

To start developing in the Croner project:

1. Clone the repository.
2. Navigate into the project directory.
3. Build the project using cargo build.
4. Run tests with cargo test --all-features.
5. Run demo with cargo run --example simple_demo

Contributing

We welcome contributions! Please feel free to submit a pull request or open an issue.

License

This project is licensed under the MIT License - see the LICENSE.md file for details.

Disclaimer

Please note that Croner is currently in its early stages of development. As such, the API is subject to change in future releases, adhering to semantic versioning principles. We recommend keeping this in mind when integrating Croner into your projects.

Contact

If you have any questions or feedback, please open an issue in the repository and we'll get back to you as soon as possible.