---
id: ciso8601
version: "2.3.3"
license: MIT
license_treatment: permissive
maintenance: active
---

# ciso8601 — Fast ISO8601 date time parser for Python written in C

License: permissive · Maintenance: active · Downloads: 11.8M/mo

## What it is and what it does

ciso8601 is a C extension that converts ISO 8601 and RFC 3339 date-time strings into Python datetime objects. It is designed as a high-performance alternative to pure-Python parsers, with no runtime dependencies beyond Python itself. The library parses a subset of ISO 8601 that covers the most common use cases—basic and extended formats with optional time zone information—and returns either a timezone-aware or naive datetime depending on the input.

The package offers strict error handling: parse_datetime() either returns a valid datetime or raises ValueError with a description of why the string doesn't conform to the supported subset. It is tested across Python 2.7 and 3.4–3.14, though the C extension requires pre-built wheels or a compiler at install time. The documentation notes that since Python 3.11, the built-in datetime module has improved significantly, and ciso8601 may not be necessary for all use cases.

Use it for:

- Parse timestamps in high-throughput APIs or data pipelines where nanosecond-scale performance gains matter.
- Deserialize ISO 8601 timestamps from JSON or other structured data formats in performance-critical applications.
- Handle RFC 3339 timestamps from web services or logs when speed is a requirement.
- Replace slower pure-Python date parsers in existing codebases without changing the calling interface.

## Worth the install?

AI-flagged interpretation of the facts above — verify before relying.

Parses ISO 8601 and RFC 3339 date-time strings into Python datetime objects using a C implementation optimized for speed.

Yes, if you parse ISO 8601 or RFC 3339 strings in a performance-sensitive context and can tolerate the C extension dependency. The compiled wheels cover common platforms, and the library is actively maintained. However, if you're on Python 3.11+ and performance is not critical, the built-in datetime module may be sufficient and simpler. No known security vulnerabilities.

## Install

pip install ciso8601
uv add ciso8601
poetry add ciso8601

## Installing ciso8601

Before you install: Medium install friction due to compiled C extension requiring platform-specific wheels. Wheels are available for common platforms (macOS, Linux, Windows) across Python 3.10–3.12, but installation may require a C compiler on unsupported architectures. Maintenance is active with recent releases.

License in practice: MIT license is permissive and places no restrictions on use, modification, or distribution in proprietary or open-source projects.

Quickstart:

pip install ciso8601

import ciso8601
dt = ciso8601.parse_datetime('2014-12-05T12:30:45.123456-05:30')
print(dt)  # datetime.datetime(2014, 12, 5, 12, 30, 45, 123456, tzinfo=...)

Requires a C compiler or pre-built wheel for your platform; some architectures may not have wheels available.

Verify before relying:

- Whether backports.datetime_fromisoformat or the built-in datetime module in Python 3.11+ are sufficient for your use case instead.
- Exact performance gains in your specific workload; benchmarks show nanosecond-scale differences that may not matter outside tight loops.

## Package facts

- License: MIT (permissive)
- Python support: unspecified
- Install friction: medium
- Maintenance: active
- Downloads: 11.8M/month (top 5,000 on PyPI)
- Known vulnerabilities: none known

## Tags

iso 8601 date parser, rfc 3339 datetime parsing, fast date string to datetime, c extension datetime parser, iso datetime converter, datetime-parsing, performance-critical

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