How to calculate the on device time.

[FlorencDemrozi]

Hi community,
We are using the Nordic Thigy:52 devices to collect inertial data that are saved on the smartphone (and transmitted to the cloud).
We have a timestamp (t_i) for each received package corresponding to the packet reception time. However (as we all know), the data contained into that package was generated over the Thingy52 at the time (t_i - X).
Has anyone faced such an issue and knows how we can calculate the value of X or the on-device time in general?

Also links to older/closed issues are more than welcome.

Thanks.