denoland
`Deno.cron` Day-of-Week mismatch: Numeric weekdays off by one
Version: Deno 1.38.5
There is an inconsistency with the numeric representation of weekdays in the Deno.cron function, it is off by one compared to standards.
Expected Behavior:
Based on conventional system cron logic (as detailed in the references listed below), the expression * * * * 3 should denote every minute of every Wednesday. This follows the typical numbering of days where Sunday is 0, Monday is 1, and so forth.
References:
- Wikipedia's cron overview
- POSIX crontab
- IBM's documentation of UNIX Cron
- Kubernetes documentation of Schedule syntax
- The popular site Crontab.guru
Actual Behavior:
In Deno, the expression * * * * 4 is required to trigger events every minute on Wednesdays. This originates from the Rust crate saffron.
Saffron, as explained in a Cloudflare blog post, is influenced by the (Java) Quartz scheduler's cron parser. Quartz adopts a different standard where the week starts with 1 (Sunday) and ends with 7 (Saturday). This is also documented in saffron issue #19.
Concern:
The main concern is the adoption of Quartz's weekday numbering in Deno, which contradicts the more widely accepted system cron standard (0 for Sunday, 1 for Monday, etc.).
Alternative Solution:
In response to this (and other issues with existing Rust cron implementations), I have created an alternative crate, croner, which aims to align more closely with the standard cron behavior, and extend them with commonly adopted additional features. A detailed comparison between croner and the existing cron and saffron crates can be found at Why Croner instead of Cron or Saffron? (Disclaimer: This comparison is based on my own quick tests, as documentation for the other crates is limited.).
From version 2.0.0, croner defaults to POSIX-style, as it did in 1.0.0, but can optionally switch to Quartz-mode using the builder method .with_alternative_weekdays(). I'd really like to see Deno.cron use POSIX style by default, but have the option to switch to Quartz mode using an option in Deno.cron.
I just noticed that your documentation points to crontab.guru, which uses different weekday numbering compared to Deno.
