cron parser

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Library for parsing cron syntax, returning the next available date.

Example:

use chrono::{TimeZone, Utc};
use cron_parser::parse;

fn main() {
   if let Ok(next) = parse("*/5 * * * *", Utc::now()) {
        println!("when: {}", next);
   }

   // passing a custom timestamp
   if let Ok(next) = parse("0 0 29 2 *", Utc.timestamp(1893456000, 0)) {
        println!("next leap year: {}", next);
        assert_eq!(next.timestamp(), 1961625600);
   }

   assert!(parse("2-3,9,*/15,1-8,11,9,4,5 * * * *", Utc::now()).is_ok());
   assert!(parse("* * * * */Fri", Utc::now()).is_err());
}

Cron table:

```

┌───────────────────── minute (0 - 59)

│ ┌─────────────────── hour (0 - 23)

│ │ ┌───────────────── dom (1 - 31) day of month

│ │ │ ┌─────────────── month (1 - 12)

│ │ │ │ ┌───────────── dow (0 - 6 or Sun - Sat) day of week (Sunday to Saturday)

│ │ │ │ │

│ │ │ │ │

│ │ │ │ │

* * * * * command to execute

```

|Field|Required|Allowed values|Allowed special characters| |-----|--------|--------------|--------------------------| |Minutes| Yes | 0–59 | * , - / | |Hours | Yes | 0–23 | * , - / | |Day of month| Yes | 1–31 | * , - / | |Month | Yes | 1–12 | * , - / | |Day of week | Yes | 0–6 or Sun-Sat | * , - / |

For the day of the week, when using a Weekday (Sun-Sat) the expression */Day is not supported instead use the integer, reasons for this is that for example */Wed = */3 translates to run every 3rd day of week, this means Sunday, Wednesday, Saturday.