The timestamp value sent over the Client interface is similar to the timestamp in RTDE.
It is the time elapsed since the controller (software process) was started, not the “world time”.
Hence, you cannot convert this to day/month/year etc.
While the RTDE (30004) and real time client (30003) return the timestamp in nice, easy to understand fractional seconds since power up, the timestamp values from the secondary client (30002) are in number of time slices (8ms) when connected to a physical robot. When connected to a simulated robot the timestamp value is in number of microseconds (1us) since power up.