# Changing Time stamps from Monotonic to Real-Time

Source: https://forum.modalai.com/topic/4750/changing-time-stamps-from-monotonic-to-real-time
Category: VOXL 2 and VOXL 2 Mini (https://forum.modalai.com/category/26/voxl-2-and-voxl-2-mini)
Tags: time, imu, camera, voxl-2
Posted: 2025-09-18 17:16:01 UTC by Aaron Porter
Replies: 5 · Views: 1875

## Aaron Porter · 2025-09-18 17:16:01 UTC

Hello,

  This is probably more of a question that is probably designed to make everything on the voxl2 stop working correctly but I am going to ask it anyways. 

What would happen if we changed the code to use the real-time clock instead of Monotonic for voxl-camera-server and the voxl-imu-server?

Would the other services not function correctly? Would they have to also be changed to Real-time clocks? Is everything going to work smoothly like before?

Not sure where IMU-server is located, otherwise I would try it out and post here but I see Camera server can access real-time but it is not used.

Thank you for your time,
Aaron Porter

## Reply by Eric Katzfey (ModalAI staff) · 2025-09-18 17:56:05 UTC

@Aaron-Porter Why would you want to do that?

## Reply by Aaron Porter · 2025-09-18 18:56:43 UTC (in reply to Eric Katzfey)

@Eric-Katzfey 

   A second device use NTP + PPS for time sync. the problem is that the second device uses the Real-Time clock when time stamping its data. The goal is to sync up the data from the second device with the camera-server, and IMU data generated but the VOXL2. The Problem is that camera-server and imu-server use Monotonic. There is an obvious offset between real-time and monotonic but that varies from the offset +/- 150 ms. This wouldn't be a huge deal but using that data with that level of deviation cause problems with the postprocessing. 

So the current thought is if it is possible to just use the real-time instead of monotonic then there will still be an offset but should be far more consistent especially when using Pulse-per-second (PPS).

Thank you,
Aaron Porter

## Reply by Eric Katzfey (ModalAI staff) · 2025-09-18 21:25:08 UTC (in reply to Aaron Porter)

@Aaron-Porter Using CLOCK_REALTIME can be bad because it can move around. That can lead to errors when the clock jumps and it makes it seem like a deadline has passed that hasn't, etc. So, it would be much better if the application using CLOCK_REALTIME switches to CLOCK_MONOTONIC instead.

## Reply by Aaron Porter · 2025-09-19 17:59:40 UTC (in reply to Eric Katzfey)

@Eric-Katzfey 

Sorry for the delay I had to do some research since I didn't fully understand how deadlines for Real Time clocks worked and why a Real-Time Clock might skip/jump other than for reasons like Daylight savings. 

  Basically since RTC is known for jumping ahead from time to time this will cause other voxl-services to believe that they are late one performing a task but the clock just jumped ahead. If we are not using a voxl2 connected to the internet or GPS then this shouldn't pose a problem? The reason I am asking is because the information I did find pointed to jumps being cased by NTP, Battery problems, or temperature. 

I am guessing the most likely cause for a jump is temperature.

Thank you for your time,
Aaron Porter

## Reply by Eric Katzfey (ModalAI staff) · 2025-09-19 23:13:52 UTC (in reply to Aaron Porter)

@Aaron-Porter Most likely jump is due to NTP aligning the clock. The other device that is time stamping using real time clock really should move to monotonic.