Tcl Source Code

Citing the "FREE FORM SCAN" section in clock(n) from core-8-6-branch

   ISO 8601 point-in-time
      An  ISO  8601 point-in-time specification, such as “CCyymmddThh‐
      mmss,” where T is the literal “T”, “CCyymmdd hhmmss”, or “CCyym‐
      mddThh:mm:ss”.  Note that only these three formats are accepted.
      The command does not accept  the  full  range  of  point-in-time
      specifications  specified in ISO8601.  Other formats can be rec‐
      ognized by giving an explicit -format option to the  clock  scan
      command.

it seems to be some undefined behavior you're observing here regarding
the presence of the -gmt option.