Hi Stathis,
The timestamp reference point can be considered an arbitrary point in time. There is no way to set it to a custom timestamp. What you would need to do is to create some small utility function for handling this yourself.
For example: save the first received timestamp and at the same time save a reference timestamp based on your desired time measuring mechanism. For every new gaze data received, you can then subtract the first gaze data timestamp from the current gaze data timestamp and know how many microseconds has passed from your reference timestamp.
The timestamp and timesync function prototypes are there for some specific experimental purposes and there is no plan to document or examplify the usage.