⏰ Whenever¶

Typed and DST-safe datetimes for Python, available in Rust or pure Python.

Do you cross your fingers every time you work with Python’s datetime—hoping that you didn’t mix naive and aware? or that you avoided its other pitfalls? There’s no way to be sure…

✨ Until now! ✨

Whenever helps you write correct and type checked datetime code, using well-established concepts from modern libraries in other languages. It’s also way faster than other third-party libraries, and usually the standard library as well. Don’t buy the Rust hype?—don’t worry: a pure Python version is available as well.

Shows a bar chart with benchmark results.

Parse, normalize, compare to now, shift, and change timezone (1M times)

⚠️ Note: A 1.0 release is expected this year. Until then, the API may change as we gather feedback and improve the library. Leave a ⭐️ on GitHub if you’d like to see how this project develops!

Why not the standard library?¶

Over 20+ years, Python’s datetime has grown out of step with what you’d expect from a modern datetime library. Two points stand out:

It doesn’t always account for Daylight Saving Time (DST). Here is a simple example:
```
bedtime = datetime(2023, 3, 25, 22, tzinfo=ZoneInfo("Europe/Paris"))
full_rest = bedtime + timedelta(hours=8)
# It returns 6am, but should be 7am—because we skipped an hour due to DST!
```
Note this isn’t a bug, but a design decision that DST is only considered when calculations involve two timezones. If you think this is surprising, you are not alone.
Typing can’t distinguish between naive and aware datetimes. Your code probably only works with one or the other, but there’s no way to enforce this in the type system!
```
# Does this expect naive or aware? Can't tell!
def schedule_meeting(at: datetime) -> None: ...
```

Why not other libraries?¶

There are two other popular third-party libraries, but they don’t (fully) address these issues. Here’s how they compare to whenever and the standard library:

	Whenever	datetime	Arrow	Pendulum
DST-safe	✅	❌	❌	⚠️
Typed aware/naive	✅	❌	❌	❌
Fast	✅	✅	❌	❌

Arrow is probably the most historically popular 3rd party datetime library. It attempts to provide a more “friendly” API than the standard library, but doesn’t address the core issues: it keeps the same footguns, and its decision to reduce the number of types to just one (arrow.Arrow) means that it’s even harder for typecheckers to catch mistakes.

Pendulum arrived on the scene in 2016, promising better DST-handling, as well as improved performance. However, it only fixes some DST-related pitfalls, and its performance has significantly degraded over time. Additionally, it’s in a long maintenance slump with only two releases in the last four years, while many serious and long-standing issues remain unaddressed.

Why use whenever?¶

🌐 DST-safe arithmetic
🛡️ Typesafe API prevents common bugs
✅ Fixes issues arrow/pendulum don’t
⚖️ Based on proven and familiar concepts
⚡️ Unmatched performance
💎 Thoroughly tested and documented
📆 Support for date arithmetic
⏱️ Nanosecond precision
🦀 Rust!—but with a pure-Python option
🚀 Supports per-interpreter GIL

Quickstart¶

>>> from whenever import (
...    # Explicit types for different use cases
...    Instant,
...    ZonedDateTime,
...    PlainDateTime,
... )

# Identify moments in time, without timezone/calendar complexity
>>> now = Instant.now()
Instant(2024-07-04 10:36:56Z)

# Simple, explicit conversions
>>> now.to_tz("Europe/Paris")
ZonedDateTime(2024-07-04 12:36:56+02:00[Europe/Paris])

# A 'naive' datetime can't accidentally mix with other types.
# You need to explicitly convert it and handle ambiguity.
>>> party_invite = PlainDateTime(2023, 10, 28, hour=22)
>>> party_invite.add(hours=6)
Traceback (most recent call last):
  ImplicitlyIgnoringDST: Adjusting a local datetime implicitly ignores DST [...]
>>> party_starts = party_invite.assume_tz("Europe/Amsterdam")
ZonedDateTime(2023-10-28 22:00:00+02:00[Europe/Amsterdam])

# DST-safe arithmetic
>>> party_starts.add(hours=6)
ZonedDateTime(2023-10-29 03:00:00+01:00[Europe/Amsterdam])

# Comparison and equality
>>> now > party_starts
True

# Rounding and truncation
>>> now.round("minute", increment=15)
Instant(2024-07-04 10:30:00Z)

# Formatting & parsing common formats (ISO8601, RFC3339, RFC2822)
>>> now.format_rfc2822()
"Thu, 04 Jul 2024 10:36:56 GMT"

# If you must: you can convert to/from the standard lib
>>> now.py_datetime()
datetime.datetime(2024, 7, 4, 10, 36, 56, tzinfo=datetime.timezone.utc)

Read more in the feature overview or API reference.

Roadmap¶

🧪 0.x: get to feature-parity, process feedback, and tweak the API:
- ✅ Datetime classes
- ✅ Deltas
- ✅ Date and time of day (separate from datetime)
- ✅ Implement Rust extension for performance
- 🚧 Tweaks to the delta API
🔒 1.0: API stability and backwards compatibility
- 🚧 Customizable parsing and formatting
- 🚧 Intervals
- 🚧 Ranges and recurring times
- 🚧 Parsing leap seconds

Limitations¶

Supports the proleptic Gregorian calendar between 1 and 9999 AD
Timezone offsets are limited to whole seconds (consistent with IANA TZ DB)
No support for leap seconds (consistent with industry standards and other modern libraries)

Versioning and compatibility policy¶

Whenever follows semantic versioning. Until the 1.0 version, the API may change with minor releases. Breaking changes will be meticulously explained in the changelog. Since the API is fully typed, your typechecker and/or IDE will help you adjust to any API changes.

License¶

Whenever is licensed under the MIT License. The binary wheels contain Rust dependencies which are licensed under similarly permissive licenses (MIT, Apache-2.0, and others). For more details, see the licenses included in the distribution.

Acknowledgements¶

Whenever stands on the shoulders of giants:

Its core datamodel takes big inspiration from Noda Time, which in turn is inspired by the influential Joda Time.
Whenever also takes a lot of inspiration from the Temporal proposal for JavaScript. After years of design work and gathering feedback, TC39 has come up with an extraodrinarily well-specified API fit for a dynamic language.
Whenever gratefully makes use of datetime’s robust and cross-platform handling of the system local timezone.
The pure-Python version of whenever also makes extensive use of Python’s datetime and zoneinfo libraries internally.
Whenever also borrows a few nifty ideas from Jiff: A modern datetime library in Rust which takes inspiration from Temporal.
The benchmark comparison graph is adapted from the Ruff project.