fastdateinfer

Fast, consensus-based date format inference written in Rust with Python bindings.

Why?

The problem: Is 01/02/2025 January 2nd or February 1st?

Library	Approach	Problem
pandas	dayfirst=True hint	You must know the format
dateutil	Guess per-element	Inconsistent results
hidateinfer	Consensus voting	Correct, but slow

The solution: If your data contains 15/03/2025, we know it's DD/MM/YYYY (15 can't be a month). This insight applies to ALL dates, resolving ambiguous ones like 01/02/2025.

fastdateinfer implements this consensus algorithm in Rust — 270x faster than hidateinfer.

Installation

pip install fastdateinfer

Quick Start

import fastdateinfer

# Infer format from dates
result = fastdateinfer.infer(["15/03/2025", "01/02/2025", "28/12/2025"])
print(result.format)      # %d/%m/%Y
print(result.confidence)  # 1.0

# Just get the format string
fmt = fastdateinfer.infer_format(["2025-01-15", "2025-03-20"])
print(fmt)  # %Y-%m-%d

# Use with pandas
import pandas as pd
dates = ["15/03/2025", "01/02/2025", "28/12/2025"]
fmt = fastdateinfer.infer_format(dates)
df = pd.to_datetime(dates, format=fmt)

Handling Dirty Data

Real-world data is messy. fastdateinfer tolerates common issues:

# Empty strings, "N/A", trailing spaces — all handled gracefully
dates = ["15/03/2025", "20/04/2025", "", "N/A", "25/12/2025 "]
result = fastdateinfer.infer(dates)
print(result.format)      # %d/%m/%Y
print(result.confidence)  # 0.6 (reduced proportionally to dirty rows)

As long as >50% of rows share the same token structure, inference succeeds. Outliers are filtered and confidence is reduced proportionally.

Strict Mode

For pipelines where every row must conform:

# Raises ValueError if ANY date doesn't match
try:
    result = fastdateinfer.infer(
        ["15/03/2025", "20/04/2025", "not-a-date"],
        strict=True
    )
except ValueError as e:
    print(e)  # strict validation failed: 1 of 3 dates incompatible

Benchmarks

Python API (Apple Silicon)

Dates	infer()	strict=True
100	0.05 ms	0.09 ms
1,000	0.47 ms	0.84 ms
10,000	0.80 ms	4.48 ms
100,000	4.06 ms	—
1,000,000	36.7 ms	—

infer_batch (100 columns, 3 dates each): 0.22 ms — columns processed in parallel with GIL released.

Rust Core (Criterion)

Dates	Time
100	43 µs
1,000	436 µs
10,000	518 µs
100,000	1.2 ms

Pre-scan overhead is negligible — adds < 5% to large-dataset inference.

Scaling

Dates	Time	Per-date
1,000	0.47 ms	0.47 µs
10,000	0.80 ms	0.08 µs
100,000	4.06 ms	0.04 µs
1,000,000	36.7 ms	0.04 µs

Performance is sublinear due to smart sampling — only ~1000 dates are fully analyzed regardless of input size. A lightweight pre-scan ensures disambiguating dates (value > 12) are always included in the sample.

Supported Formats

Format	Example	Output
European	15/03/2025	%d/%m/%Y
American	03/15/2025	%m/%d/%Y
ISO 8601	2025-03-15	%Y-%m-%d
ISO datetime	2025-03-15T10:30:00	%Y-%m-%dT%H:%M:%S
Month name	15 Mar 2025	%d %b %Y
Month name (full)	15 March 2025	%d %B %Y
Month first	Mar 15, 2025	%b %d, %Y
Weekday + timezone	Mon Jan 13 09:52:52 MST 2014	%a %b %d %H:%M:%S %Z %Y
2-digit year	15/03/25	%d/%m/%y
With time	15/03/25 10.30.00	%d/%m/%y %H.%M.%S
Month-year only	March, 2025	%B, %Y
Day-month only	15/Mar	%d/%b

API Reference

infer(dates, prefer_dayfirst=True, min_confidence=0.0, strict=False)

Infer date format from a list of date strings.

Arguments:

• dates: List of date strings
• prefer_dayfirst: Use DD/MM for fully ambiguous dates (default: True)
• min_confidence: Minimum confidence threshold (default: 0.0)
• strict: Raise error if any date doesn't match (default: False)

Returns: InferResult with:

• format: strptime format string
• confidence: float between 0.0 and 1.0
• token_types: list of resolved token types

result = fastdateinfer.infer(["01/02/2025", "03/04/2025"], prefer_dayfirst=False)
print(result.format)  # %m/%d/%Y (American format)

infer_format(dates, prefer_dayfirst=True)

Convenience function that returns only the format string.

fmt = fastdateinfer.infer_format(["2025-01-15", "2025-03-20"])
print(fmt)  # %Y-%m-%d

infer_batch(columns, prefer_dayfirst=True)

Infer formats for multiple columns at once. Columns are processed in parallel (GIL released).

results = fastdateinfer.infer_batch({
    "transaction_date": ["15/03/2025", "01/02/2025"],
    "created_at": ["2025-01-15T10:30:00", "2025-01-16T14:45:00"],
    "value_date": ["15-Mar-2025", "01-Feb-2025"]
})

for col, result in results.items():
    print(f"{col}: {result.format}")
# transaction_date: %d/%m/%Y
# created_at: %Y-%m-%dT%H:%M:%S
# value_date: %d-%b-%Y

How It Works

1. Tokenize: Split "15/03/2025" into [15, /, 03, /, 2025]
2. Constrain: 15 can only be Day (>12), 03 could be Day or Month, 2025 is Year
3. Vote: Across all dates, count evidence for each position
4. Resolve: Position 1 has strong Day evidence → Position 2 must be Month
5. Format: Output %d/%m/%Y

The key insight: consensus converges quickly. Even with 1 million dates, we only need to analyze ~1000 to determine the format with high confidence.

Use Cases

CSV/Data Processing

import pandas as pd
import fastdateinfer

# Read raw data
df = pd.read_csv("data.csv")

# Detect format automatically
fmt = fastdateinfer.infer_format(df["date"].dropna().tolist())

# Parse with detected format
df["date"] = pd.to_datetime(df["date"], format=fmt)

Multi-format Data Pipeline

# Different columns may have different formats
results = fastdateinfer.infer_batch({
    col: df[col].dropna().astype(str).tolist()
    for col in ["date", "value_date", "created_at"]
})

for col, result in results.items():
    df[col] = pd.to_datetime(df[col], format=result.format)

Validation

# Ensure high confidence
result = fastdateinfer.infer(dates, min_confidence=0.9)
if result.confidence < 0.9:
    raise ValueError(f"Low confidence: {result.confidence}")

Comparison

Feature	fastdateinfer	hidateinfer	pandas	dateutil
Consensus-based	✅	✅	❌	❌
Speed (10k dates)	0.80 ms	200 ms	2 ms*	N/A
Dirty data tolerance	✅	❌	❌	❌
Strict validation	✅	❌	❌	❌
Returns strptime format	✅	✅	❌	❌
Parallel batch inference	✅	❌	❌	❌
Type hints	✅	❌	✅	✅
Pure Rust core	✅	❌	❌	❌

*pandas time is for parsing only (you must already know the format)

Building from Source

# Prerequisites
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
pip install maturin

# Clone and build
git clone https://github.com/coledrain/fastdateinfer
cd fastdateinfer
maturin develop --release

# Run tests
cargo test

License

MIT License. See LICENSE for details.

Contributing

Contributions welcome! Please open an issue or PR on GitHub.

Acknowledgments

• Inspired by hidateinfer
• Built with PyO3 for Python bindings
• Built for high-volume data processing pipelines