hron

Human-readable cron — scheduling expressions that are a superset of what cron can express.

every weekday at 9:00 except dec 25, jan 1 until 2027-12-31 in America/New_York

hron is a language specification with native implementations for multiple programming languages. It handles everything cron can do and more: multi-week intervals, ordinal weekdays, yearly schedules, exception dates, end dates, and IANA timezone support with full DST awareness.

hron.io — interactive playground, no install needed.

cargo install hron-cli

$ hron "every weekday at 9:00 in America/New_York"
2026-02-09T09:00:00-05:00[America/New_York]

$ hron "every weekday at 9:00" -n 3
2026-02-09T09:00:00+00:00[UTC]
2026-02-10T09:00:00+00:00[UTC]
2026-02-11T09:00:00+00:00[UTC]

$ hron "every day at 9:00" --to-cron
0 9 * * *

$ hron --from-cron "*/30 * * * *"
every 30 min from 00:00 to 23:59

$ hron --explain "0 9 * * 1-5"
every weekday at 09:00

See hron-cli for all options.

Language	Package	Registry
Rust	hron
JS/TS (native)	hron-ts
JS/TS (WASM)	hron-wasm
Dart/Flutter	hron
Python	hron
Go	hron
Java	hron
C#	Hron
Ruby	hron

Note: The JS/TS native package (hron-ts) uses the Temporal API via polyfill. Once Temporal ships natively in runtimes, performance improves automatically. For performance-critical JS/TS use cases, consider the WASM package (hron-wasm).

hron is available as a library in multiple languages:

# Rust
cargo add hron

# TypeScript / JavaScript
npm install hron-ts    # native
npm install hron-wasm  # WASM

# Dart / Flutter
dart pub add hron

# Python
pip install hron

# Go
go get github.com/prasrvenkat/hron/go

# Java (Maven) — add io.hron:hron to pom.xml (see badge above for latest version)

# C# / .NET
dotnet add package Hron

# Ruby
gem install hron

See language-specific READMEs for API docs and examples: Rust · TypeScript · Dart · Python · Go · Java · C# · Ruby · WASM

every day at 09:00
every weekday at 9:00
every weekend at 10:00
every monday at 9:00
every mon, wed, fri at 9:00

every 30 min from 09:00 to 17:00
every 2 hours from 00:00 to 23:59
every 45 min from 09:00 to 17:00 on weekdays

every 2 weeks on monday at 9:00
every 3 weeks on mon, wed at 10:00

every month on the 1st at 9:00
every month on the 1st, 15th at 9:00
every month on the last day at 17:00
every month on the last weekday at 15:00
every month on the first monday at 10:00
every month on the last friday at 16:00
every month on the third thursday at 11:00
every month on the nearest weekday to 15th at 9:00
every month on the next nearest weekday to 1st at 9:00
every month on the previous nearest weekday to 31st at 17:00

every year on dec 25 at 00:00
every year on jul 4 at 12:00
every year on the first monday of march at 10:00
every year on the third thursday of november at 12:00
every year on the 15th of march at 09:00
every year on the last weekday of december at 17:00
every year on the last friday of december at 17:00

on feb 14 at 9:00
on 2026-03-15 at 14:30

Trailing clauses can be combined in this order: except, until, starting, during, in.

every weekday at 9:00 except dec 25, jan 1
every weekday at 9:00 except 2026-07-04
every day at 09:00 until 2026-12-31
every day at 09:00 until dec 31
every 2 weeks on monday at 9:00 starting 2026-01-05
every weekday at 9:00 in America/New_York
every day at 9:00 during jan, jun
every weekday at 9:00 except dec 25 until 2027-12-31 during jan, dec in UTC

• except — skip specific dates. Named dates (dec 25) recur every year. ISO dates (2026-07-04) are one-off.
• until — stop producing occurrences after this date.
• starting — anchor date for multi-week intervals.
• during — only fire during specific months.
• in — IANA timezone. Must be last.

hron can convert to and from standard 5-field cron expressions for the expressible subset:

Expressions that go beyond cron's capabilities (multi-week intervals, ordinals, yearly, except, until, partial-day windows) will return an error from to_cron().

When a schedule specifies a timezone via the in clause, all occurrences are computed in that timezone with full DST awareness:

• Spring-forward (gap): If a scheduled time doesn't exist (e.g. 2:30 AM during a spring-forward transition), the occurrence shifts to the next valid time after the gap (typically 3:00 AM or later, depending on the gap size).
• Fall-back (ambiguity): If a scheduled time is ambiguous (e.g. 1:30 AM occurs twice during fall-back), the first (pre-transition) occurrence is used.
• No timezone: When no in clause is specified, UTC is used for deterministic, portable behavior.

All implementations (Rust, TypeScript, Dart, Python, Go, Java, C#, Ruby, WASM) follow these same DST semantics.

The conformance test suite includes explicit spring-forward and fall-back test cases to verify this behavior across all implementations.

The spec/ directory contains the language-agnostic conformance test suite (tests.json) and formal grammar (grammar.ebnf). The grammar is a reference specification — all parsers are hand-written recursive descent, not generated from the EBNF. All language implementations must pass the conformance tests.

Requires just.

just test-all        # Run tests across all languages
just test-rust       # Run Rust tests only
just test-ts         # Run TypeScript tests only
just test-dart       # Run Dart tests only
just test-python     # Run Python tests only
just test-go         # Run Go tests only
just test-java       # Run Java tests only
just test-csharp     # Run C# tests only
just test-ruby       # Run Ruby tests only
just build-wasm      # Build WASM package
just bench           # Run Criterion benchmarks (Rust)
just fuzz            # Run fuzz targets (requires nightly, default 3 min)
just stamp-versions  # Stamp VERSION into all package manifests

MIT