The GST start epoch is 0h UTC on Sunday, 22 August 1999 (midnight between 21 and 22 August).
Date and time
WN
ToW
GST Epoch
GST Epoch
WN
ToW
Date and time
The GST is given as 32-bit binary number composed of two parameters as follows:
Source: Galileo Open Service Signal In Space Interface Control Document (OS SIS ICD), GSA, Issue 1.1, September 2010
WN - 12bits The Week Number is an integer counter that gives the sequential week number from the origin of the Galileo time.
This parameter is coded on 12 bits, which covers 4096 weeks (about 78 years).
Then the counter is reset to zero to cover additional period modulo 4096.
TOW - 20bits The Time of Week is defined as the number of seconds that have occurred since the transition from the previous week.
The TOW covers an entire week from 0 to 604799 seconds and is reset to zero at the end of each week.