This is a wrapper around clock scan for Tcl older than 8.6, which lacks the -format argument to manually specify a timestamp format. This will remove any timezone offset, which clock scan will choke on, and turns it into a relative time offset to apply to the remaining timestamp.

clock_scan.tcl

proc urllog:clock_scan {datestring} {
  set regpat {([-+]{1}[0-9]{1,4})}
  set offset_string "0 seconds"
  if {[regexp $regpat $datestring -> offset]} {
    regsub $regpat $datestring {} datestring
    set offset_hours 0
    set offset_minutes 0
    set offset_length [string length $offset]
    scan $offset %d offset
    set offset_sign [expr { ($offset > 0) ? "ago" : "" }]
    if {$offset < 0} { set offset [expr 0 - $offset] }
    if {$offset_length == 5} {
      set offset_hours [expr $offset / 100]
      set offset_minutes [expr $offset % 100]
    } else {
      set offset_hours $offset
    }
    set offset_string "$offset_hours hours $offset_minutes minutes $offset_sign"
  }
  return [clock scan $offset_string -base [clock scan $datestring]]
}