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arXiv · 2609.36931

Dating the Model: Hidden Dates in System Prompts Affect LLM Evaluation

Mario Sanz-Guerrero·Minh Duc Bui·Manuel Mager·Katharina von der Wense
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Abstract

Reproducibility is essential for scientific research, yet prior work shows that LLM outputs vary with hardware and batching. We identify an overlooked factor: the hidden injection of the current date into system prompts, which users cannot control and which changes every day. Across 9 recent LLMs and 6 datasets spanning multiple-choice QA (MCQA), math reasoning, code generation, and machine translation, performance varies solely with the current date, with deltas of up to 6% on MCQA, 14% on math reasoning, 7% on code generation, and 2.84 BLEU on machine translation. Model rankings also shift, affecting leaderboards. This date effect exceeds other sources of non-determinism, such as batch size and numerical precision. Standard prompting techniques -- chain-of-thought and few-shot prompting -- do not reduce the sensitivity; chain-of-thought even amplifies it. Our findings underscore the need for careful evaluation protocols to ensure reproducibility and fair comparisons in LLM research.

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Mario Sanz-Guerrero, Minh Duc Bui, Manuel Mager, Katharina von der Wense. 2026-09-29. Dating the Model: Hidden Dates in System Prompts Affect LLM Evaluation. https://arxiv.org/abs/2609.36931

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