Is microsecond precision available in an iRule?

On my 10.1.0 box, "clock clicks" returns microseconds. Here's an example:

     set a1 [clock clicks]
     set b1 [clock clicks -milliseconds]
     after 10
     set a2 [clock clicks]
     set b2 [clock clicks -milliseconds]
     after 100
     set a3 [clock clicks]
     set b3 [clock clicks -milliseconds]
     after 1000
     set a4 [clock clicks]
     set b4 [clock clicks -milliseconds]
     log local0. "As: $a1 $a2 $a3 $a4"
     log local0. "Bs: $b1 $b2 $b3 $b4"
     log local0. "a2-a1: [expr $a2 - $a1]  a3-a2: [expr $a3 - $a2]  a4-a3: [expr $a4 - $a3]"
     log local0. "b2-b1: [expr $b2 - $b1]  b3-b2: [expr $b3 - $b2]  b4-b3: [expr $b4 - $b3]"

and here's what it logs:

 As: -1076552924 -1076543381 -1076442893 -1075443255
 Bs: -426278316 -426278306 -426278206 -426277206
 a2-a1: 9543  a3-a2: 100488  a4-a3: 999638
 b2-b1: 10  b3-b2: 100  b4-b3: 1000

The microseconds counts each round to the number of milliseconds that we waited, and that were measured.

The negative numbers are because Tcl is presenting unsigned numbers as signed numbers, but the subtraction math still works out, so don't be fooled by that. However, this also means you won't be able to measure durations of more than 2^31 microseconds, which is about 35 minutes.