Time Date Interfaces processes day SPN (962) wrong

[MxmlnStck]

Describe the bug
The time date interface is calculating the day wrong. According to the specs https://www.isobus.net/isobus/pGNAndSPN/16421?type=PGN the implementation is correct, but the signal always must be rounded up.

Supporting Documentation
I'm using a ros2 node to request the time and date from the ISOBUS. My system timestamp resolves to the 4th of December, the interface is returning the 3rd of December:
[1764830769.779073577] [can.isobus_interface_node]: Received ISOBUS time/date: 2025-12-03 07:49:05 (from address 240)

Environment (please complete the following information):

• OS: [Ubuntu 24.04.3]
• Compiler [GCC 11.4.0]
• CAN Driver [Socket CAN]

[ad3154]

Hey there, apologies for the delay in looking at this.

I am not 100% sure I am understanding, so I just want to clarify... can you be a bit more specific on where the issue is? Are you saying it's our send_time_and_date function in the TimeDateInterface, or are you receiving it on the bus and we should be rounding somewhere within process_rx_message? Or am I missing what you are getting at?

[MxmlnStck]

Hi, sorry that's on me. I've done some more logging to explain in detail.
I'm receiving the isobus time/date message and in the process_rx_message function there is a rounding mistakes. Let me explain in detail: According to the standard (https://www.isobus.net/isobus/pGNAndSPN/17065?type=PGN) the days are scaled by 0.25.

With that scaling it allows us to get days and quarter days. For today, the 10th, I'm receiving the values 37, 38, 39, 40 - depending of the time:
00:00:00 - 05:59:59 will return 37
06:00:00 - 11:59:59 will return 38
12:00:00 - 17:59:59 will return 39
18:00:00 - 23:59:59 will return 40

The process_rx_message only does days = message.get_uint8_at(4) / 4. For 37, 38, 39 this gives us a 9 instead of a 10. What we have to do instead is to ceil it: days = std::ceil(static_cast<float>(message.get_uint8_at(4)) / 4). This would return a 10 for all of the 9.xx values.

So, the fix would be (all in isobus_time_date_interface.cpp):

• add #include <cmath> for std::ceil
• change line 153 to: timeAndDateInformation.timeAndDate.day = std::ceil(static_cast<float>(message.get_uint8_at(4)) / 4); // This is SPN 962

Attached you can find some debug output from my test program. There you can see again how the return values for the day look like and how it's calculating the wrong day for the first 3 quarters of the day.

I hope that makes it more clear, otherwise let me know if you need further information.

log.txt