← Findings
LowReasoningReviewer-confirmedPublished

Date and duration arithmetic errors

Models miscompute differences between dates, weekdays, and durations across boundaries like months and leap years.

Published June 26, 2026

Reproducibility
Often
Severity
Low
Confidence
Reviewer-confirmed

Details

Questions like 'how many days between two dates' or 'what weekday was X' produce frequent off-by-N errors, especially across month/year boundaries and leap years. Reliable only when delegated to a tool.

Found with

🔬 Property-based testing

Generate date pairs; check against a real date library.

Evidence

Q: How many days from 2024-02-15 to 2024-03-15?
A: Returns 30 (correct: 29; 2024 is a leap year).
Illustrative example — see the linked reference for the documented evidence.

Affected versions

Anthropic · claude-opus-4-8OpenAI · gpt-4oGoogle · gemini-2.0-flashMeta · llama-3.3-70b

References

Reasoning failure

Source: https://arxiv.org/abs/2406.09170

Cite this

Qlarify Labs. (2026). Date and duration arithmetic errors. Retrieved from https://labs.qlarify.fi/findings/date-duration-arithmetic