The Galileo System Time  (GST) is a continuous timescale based on the definition of the second (according to the international System of Units, SI). Its origin/reference epoch GST(T0) is coincident with 1999-08-21 @ 23:59:47 UTC.

Universal Time Coordinated (UTC) is the time scale endorsed by the 15th General Conference of Weights and Measures for worldwide time coordination and dissemination. It is the international time metrological standard maintained, coordinated and disseminated by the BIPM. UTC is defined as an atomic timescale (time interval is consistent with the SI definition of the second). Still unlike GST, it is steered towards UT timescale which is not based on atomic standards but on the Earth rotation period.

Galileo System Time (GST)
Week Number
Time of Week

Universal Time Coordinated (UTC)