The paper describes our thoughts of forming the system time of a Global Navigation Satellite System (GNSS)

which is denoted by GNSST

and the time references TA(k) and UTC(k) of a timing laboratory where k characterizes the specific national laboratory.We also describe the roles of time scale algorithms

clock steering algorithms

and clock prediction algorithms in forming these time scales.We take Chinese Beidou satellite navigation system time (denoted BDT) as an example

and propose a twice steering algorithm to form BDT.We take the national time service center (NTSC) of China as an example to describe the idea

method

principle and theory of forming a TA(k) and a UTC(k).Then we analyze the impact of TA(NTSC) on UTC(NTSC)

and the impact of UTC(NTSC) on the time synchronization precision and frequency stability performances of BDT if we steered BDT to UTC(NTSC).We also propose an idea of steering BDT to a paper time scale formed from UTC(NTSC) and UTC(BSNC).The performance is theoretically analyzed.