clock clicks ?-option?
clock clicks returns a high-resolution time value as a system-dependent integer value. See the clock reference page for more detail.
If -milliseconds is provided as an option, then the command is synonomous with clock milliseconds. This usage is OBSOLETE.
If -microseconds is provided as an option, then the command is synonomous with clock microseconds. This usage is OBSOLETE.
In Classic MacOS, the clicks were typically as long as 1/60 second, but in recent MacOS X, they are instead so fast that
expr {[clock clicks] == [clock clicks]}never seems to return true (more than µs resolution). Other platforms still have had clicks = milliseconds, and this assumption is sometimes made in Tcl code.
In other words, the uniqueness of this value is notoriously unreliable, especially for cross-platform comparisons. An alternative source when generating unique IDs (cf. GUID and UUID) is to use info cmdcount.