One method that may work is below. And I kept it under 200 lines
Bill

code:
$Time = @TIME
$ClkHr  = VAL (SUBSTR ($Time, 1, 2))		; REM **  Parse HOUR value
$ClkMin = VAL (SUBSTR ($Time, 4, 2))		; REM **  Parse MINUTE value

$Bias = READVALUE ("HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\TimeZoneInformation", "ActiveTimeBias")
$Name = READVALUE ("HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\TimeZoneInformation", "StandardName")
$GmtVal = READVALUE ("HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Time Zones\$Name", "Display")

; REM **  Clock set to adjust for DayLight Savings Time.
IF ($Bias <> "0")
  $BiasVal = VAL ("$Bias")
  $OffSet = SUBSTR ("$GmtVal", 5, 1)			; REM **  Does local clock add or subtract time from GMT?
  IF ($OffSet <> "+") AND ($OffSet <> "-")
    ? "Time zone is already Greenwich Mean Time"
    GOTO EndScript
  ENDIF
ENDIF

SELECT
  CASE ($BiasVal = "+780") OR ($BiasVal = "-780")	$AdjHr = 13	$AdjMin = 0	; REM **  Plus/minus 13 hours
  CASE ($BiasVal = "+720") OR ($BiasVal = "-720")	$AdjHr = 12	$AdjMin = 0	; REM **  Plus/minus 12 hours
  CASE ($BiasVal = "+660") OR ($BiasVal = "-660")	$AdjHr = 11	$AdjMin = 0	; REM **  Plus/minus 11 hours
  CASE ($BiasVal = "+600") OR ($BiasVal = "-600")	$AdjHr = 10	$AdjMin = 0	; REM **  Plus/minus 10 hours
  CASE ($BiasVal = "+570") OR ($BiasVal = "-570")	$AdjHr = 9	$AdjMin = 30	; REM **  Plus/minus 9 hours, 30min
  CASE ($BiasVal = "+540") OR ($BiasVal = "-540")	$AdjHr = 9	$AdjMin = 0	; REM **  Plus/minus 9 hours
  CASE ($BiasVal = "+480") OR ($BiasVal = "-480")	$AdjHr = 8	$AdjMin = 0	; REM **  Plus/minus 8 hours
  CASE ($BiasVal = "+450") OR ($BiasVal = "-450")	$AdjHr = 7	$AdjMin = 30	; REM **  Plus/minus 7 hours, 30min
  CASE ($BiasVal = "+420") OR ($BiasVal = "-420")	$AdjHr = 7	$AdjMin = 0	; REM **  Plus/minus 7 hours
  CASE ($BiasVal = "+390") OR ($BiasVal = "-390")	$AdjHr = 6	$AdjMin = 30	; REM **  Plus/minus 6 hours, 30min
  CASE ($BiasVal = "+360") OR ($BiasVal = "-360")	$AdjHr = 6	$AdjMin = 0	; REM **  Plus/minus 6 hours
  CASE ($BiasVal = "+345") OR ($BiasVal = "-345")	$AdjHr = 5	$AdjMin = 45	; REM **  Plus/minus 5 hours, 45min
  CASE ($BiasVal = "+330") OR ($BiasVal = "-330")	$AdjHr = 5	$AdjMin = 30	; REM **  Plus/minus 5 hours, 30min
  CASE ($BiasVal = "+300") OR ($BiasVal = "-300")	$AdjHr = 5	$AdjMin = 0	; REM **  Plus/minus 5 hours
  CASE ($BiasVal = "+270") OR ($BiasVal = "-270")	$AdjHr = 4	$AdjMin = 30	; REM **  Plus/minus 4 hours, 30min
  CASE ($BiasVal = "+240") OR ($BiasVal = "-240")	$AdjHr = 4	$AdjMin = 0	; REM **  Plus/minus 4 hours
  CASE ($BiasVal = "+210") OR ($BiasVal = "-210")	$AdjHr = 3	$AdjMin = 30	; REM **  Plus/minus 3 hours, 30min
  CASE ($BiasVal = "+180") OR ($BiasVal = "-180")	$AdjHr = 3	$AdjMin = 0	; REM **  Plus/minus 3 hours
  CASE ($BiasVal = "+150") OR ($BiasVal = "-150")	$AdjHr = 2	$AdjMin = 30	; REM **  Plus/minus 2 hours, 30min
  CASE ($BiasVal = "+120") OR ($BiasVal = "-120")	$AdjHr = 2	$AdjMin = 0	; REM **  Plus/minus 2 hours
  CASE ($BiasVal = "+60")  OR ($BiasVal = "-60")	$AdjHr = 1	$AdjMin = 0	; REM **  Plus/minus 1 hour
  CASE ($BiasVal = "+0")   OR ($BiasVal = "-0")		$AdjHr = 0	$AdjMin = 0	; REM **  Plus/minus 0 hours
ENDSELECT

IF ($OffSet = "-")
  IF ($AdjMin > $ClkMin)	$GmtMin = 60 - ($AdjMin - $ClkMin)	$AdjHr = $AdjHr + 1
  ELSE				$GmtMin = ($ClkMin - $AdjMin)
  ENDIF
  IF ($AdjHr > $ClkHr)		$GmtHr = 24 - ($AdjHr - $ClkHr)
  ELSE				$GmtHr = ($ClkHr - $AdjHr)
  ENDIF
ELSE
  IF ($AdjMin + $ClkMin) > 60	$GmtMin = 60 - ($AdjMin + ClkMin)	$AdjHr = $AdjHr - 1
  ELSE				$GmtMin = ($AdjMin + ClkMin)
  ENDIF
  IF ($AdjHr + $ClkHr) > 24	$GmtHr = 24 - ($AdjHr + $ClkHr)
  ELSE				$GmtHr = ($AdjHr + $ClkHr)
  ENDIF
ENDIF

:EndScript