Data-to-text generation tasks often involve processing numerical time-series as input such as financial statistics or meteorological data. Although large language models (LLMs) are a powerful approach to data-to-text, we still lack a comprehensive understanding of how well they actually understand time-series data. We therefore introduce a benchmark with 18 evaluation tasks to assess LLMs’ abilities of interpreting numerical time-series, which are categorized into: 1) event detection—identifying maxima and minima; 2) computation—averaging and summation; 3) pairwise comparison—comparing values over time; and 4) inference—imputation and forecasting. Our experiments reveal five key findings: 1) even state-of-the-art LLMs struggle with complex multi-step reasoning; 2) tasks that require extracting values or performing computations within a specified range of the time-series significantly reduce accuracy; 3) instruction tuning offers inconsistent improvements for numerical interpretation; 4) reasoning-based models outperform standard LLMs in complex numerical tasks; and 5) LLMs perform interpolation better than forecasting. These results establish a clear baseline and serve as a wake-up call for anyone aiming to blend fluent language with trustworthy numeric precision in time-series scenarios.