feat(cron) implement Deno.cron() (#21019)

This PR adds unstable `Deno.cron` API to trigger execution of cron jobs.

* State: All cron state is in memory. Cron jobs are scheduled according
to the cron schedule expression and the current time. No state is
persisted to disk.
* Time zone: Cron expressions specify time in UTC.
* Overlapping executions: not permitted. If the next scheduled execution
time occurs while the same cron job is still executing, the scheduled
execution is skipped.
* Retries: failed jobs are automatically retried until they succeed or
until retry threshold is reached. Retry policy can be optionally
specified using `options.backoffSchedule`.

1 files changed, 19 insertions, 0 deletions

diff --git a/ext/cron/time.rs b/ext/cron/time.rs
new file mode 100644
index 000000000..60375818b
--- /dev/null
+++ b/ext/cron/time.rs
@@ -0,0 +1,19 @@
+// Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
+
+/// Identical to chrono::Utc::now() but without the system "clock"
+/// feature flag.
+///
+/// The "clock" feature flag pulls in the "iana-time-zone" crate
+/// which links to macOS's "CoreFoundation" framework which increases
+/// startup time for the CLI.
+pub fn utc_now() -> chrono::DateTime<chrono::Utc> {
+  let now = std::time::SystemTime::now()
+    .duration_since(std::time::UNIX_EPOCH)
+    .expect("system time before Unix epoch");
+  let naive = chrono::NaiveDateTime::from_timestamp_opt(
+    now.as_secs() as i64,
+    now.subsec_nanos(),
+  )
+  .unwrap();
+  chrono::DateTime::from_naive_utc_and_offset(naive, chrono::Utc)
+}