openai function calling prompt for train trip planning

gistfile1.txt

{
    "messages": [
        {
            "role": "system",
            "content": "The current date and time is 2023-07-20T13:25:20+02:00. Be very assumptive when guessing the values of function parameters."
        },
        {
            "role": "user",
            "content": "am ut 6"
        }
    ],
    "model": "gpt-4-0613",
    "max_tokens": null,
    "stream": false,
    "n": 1,
    "temperature": 0.0,
    "functions": [
        {
            "name": "custom_search",
            "description": "answer all questions about NS",
            "parameters": {
                "title": "QAToolInput",
                "type": "object",
                "properties": {
                    "question": {
                        "title": "Question",
                        "description": "question about NS",
                        "type": "string"
                    }
                },
                "required": [
                    "question"
                ]
            }
        },
        {
            "name": "plan_plublic_transport_trip",
            "description": "Plan a public transport trip from A to B in the Netherlands.",
            "parameters": {
                "type": "object",
                "properties": {
                    "origin": {
                        "description": "origin station name in the Netherlands for the trip. Be assumptive and accept very short shorthands.",
                        "type": "string"
                    },
                    "destination": {
                        "description": "destination station name in the Netherlands for the trip. Be assumptive and accept very short shorthands.",
                        "type": "string"
                    },
                    "another_day_reference": {
                        "description": "True is the request refers to another day than today.",
                        "type": "boolean"
                    },
                    "trip_date_time": {
                        "description": "Requested DateTime for the departure or arrival of the trip in 'YYYY-MM-DDTHH:MM:SS+02:00' format. Do not use the current date en time, if a time or number is provided by the user. Use the number in the last user message.",
                        "type": "string"
                    },
                    "day_part_mentioned_next_to_time": {
                        "description": "True if the day part is mentioned next to the time, False otherwise.",
                        "type": "boolean"
                    },
                    "explicit_day_part_reference": {
                        "description": "Always prefer None! None if the request refers to the current day, otherwise the part of the day the request refers to.",
                        "enum": [
                            "none",
                            "morning",
                            "afternoon",
                            "evening",
                            "night"
                        ],
                        "type": "string"
                    },
                    "departure": {
                        "description": "True to depart at the given time, False to arrive at the given time",
                        "type": "boolean"
                    },
                    "language": {
                        "description": "Language of the input text",
                        "type": "string"
                    }
                },
                "required": [
                    "origin",
                    "destination",
                    "another_day_reference",
                    "trip_date_time",
                    "day_part_mentioned_next_to_time",
                    "explicit_day_part_reference",
                    "departure",
                    "language"
                ]
            }
        }
    ]
}

I added all kinds of part-of day references to repair the time of the day, but recently I also seem to have some succes with just the following:
"trip_date_time": {
                        "description": "Requested DateTime for the departure or arrival of the trip in 'YYYY-MM-DDTHH:MM:SS+02:00' format. The user will use a time in a 12 hour system, make an intelligent guess about what the user is most likely to mean in terms of a 24 hour system, e.g. not planning for the past.",
                        "type": "string"
                    },