I assume the string is really 20060831120000. When dealing with dates I would recommend you always use the clock command.
First you need your date in a format you can scan which means a space between date and time
set dt 20060831120000
regsub — {(d{8})(d+)} $dt {1 2} dt
echo [clock format [clock scan $dt -format “%m/%d/%Y, %H%M Hours”]
==> 08/31/2006, 1200 Hours
Adjust the format as needed
