'''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].

[Scott Beasley]

I feel that [info cmdcount] is totally useless for Unique key generation.  It stays very small in size for too long and changes at a MUCH lower rate then does [clock clicks] in a modern tcl (8.4-8.5) system.

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