[{'role': 'system', 'content': "You are an expert in machine learning and your task is to classify code snippets into one of the following categories: ['helper_functions', 'load_data', 'data_exploration', 'data_preprocessing', 'evaluation', 'modelling', 'prediction', 'result_visualization', 'save_results', 'comment_only']."}, {'role': 'user', 'content': 'Identify the single most appropriate category to which the provided code belongs. Provide your response as a single category label without any additional information, commentary, or personal opinions.\\nPython Code:from datetime import timedelta\n\nTEST_DAYS = 7\n\nTRAIN_LAST =  - timedelta(days=TEST_DAYS)\n\nTEST_FIRST = sub_df["Date"].min()\nTEST_DAYS = (df["Date"].max() - TEST_FIRST).days + 1\n\ndev_df, test_df = df[df["Date"] < TEST_FIRST].copy(), df[df["Date"] >= TEST_FIRST].copy()\ndev_df.shape, test_df.shape'}, {'role': 'assistant', 'content': 'data_preprocessing'}]