Global Navigation Satellite Systems (GNSSs) use internal reference time scales: GPS Time, GLONASS Time, Galileo System Time and BeiDou System Time. Constructed from a clock ensemble, they are designed for internal system synchronization, necessary to produce a navigation solution. They are usually steered to an external stable reference time scale, for example UTC(USNO), modulo 1 s, for GPS time. To achieve safe operation of a GNSS, a system time should preferably be a uniform time scale not affected by the leap seconds of Coordinated Universal Time (UTC). But this is not compatible with international recommendations that radio broadcast time signals should conform as closely as possible to UTC. This paper describes the various approaches chosen by GNSS providers and the relation between GNSS system times and UTC in terms of numbering of seconds. Different solutions for numbering seconds do not help the GNSS interoperability. This paper also explains why, on some occasions, GNSS system times play a role of alternative time scales with the consequent risk of confusion.