Following article will describe shortly the hardware time stamping for the D400e camera series in a more conversational way. 

There is a 64bit counter internal to the camera FPGA, this counts up every 1us. When any function in the camera (FPGA side) requires a time stamp this counter is latched at the correct point for that function. For image capture events the time stamp is aligned to the start of the exposure of that captured stream (RGB or Stereo streams). If both are used, the RGB comes first.

If you are looking to accurately measure the interval between frames the hardware stamp will provide you with highly accurate and robust use -with global_time disabled-. If you need to have the clock in local system format to compare with other timings, please review the source file for the 'converter' global_timestamp_reader.cpp. 

The python script attached to this article was created to print the time stamp. This script illustrates both cases: when the global time is enabled and disabled, follow instructions in script to make use of each case. Please note is only an example.