[Discuss-gnuradio] usrp2: timestamp absolute ?

[Michael Sprauer]

Hello list,

my question is about the time stamp in rx_metadata->timestamp. I've got the 
rx_streaming_samples running and now I wondering what the time value is about.
In the code I've found this:
uint32_t        timestamp;      // time of rx or tx (100 MHz)

Can you tell me what the timestamp exactly refers to. Is it just a counter? 
When/How is it started/reseted?
Or to be more precisely: How do I calculate an absolute time like the extended 
unix time stamp with uSec and nSec?

Thanks for reading :)

kind regards
Michael