Time to digital converter (TDC) for time of flight (TOF) applications. 66.6 picosecond sample
Regarding 


  Does this mean that if I wait for 12ns then all samples thereafter are within 55ps accuracy. In other words. fastest sample = 12ns , next sample = 12.055ns


Actually the TDC7200 datashet specifies:

+-----------------+------------------------------+-----+-----+-----+------+  
|      Param      |         Description          | min | typ | max | unit |  
+-----------------+------------------------------+-----+-----+-----+------+  
| T1STARTSTOP_Min | t_min between Start and Stop |  12 |     |     | ns   |  
| T1STOPSTOP_Min  | t_min between 2 Stop Signals |  67 |     |     | ns   |  
+-----------------+------------------------------+-----+-----+-----+------+


table adapted from datasheet, formatted thanks to online tool

So if first sample is at 12ns then next sample cannot arrive before 12+67=79 ns.