Source code for gmt.vbs

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Option Explicit
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Dim dtmDate, dtmGMT
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Dim colItems, objItem, objWMIService
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Dim strArg, strDate
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dtmDate = Now
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If WScript.Arguments.Named.Count   > 0 Then Syntax

If WScript.Arguments.Unnamed.Count > 0 Then

	For Each strArg In WScript.Arguments.Unnamed

		strDate = Trim( strDate & " " & strArg )

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	Next
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	On Error Resume Next
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	If IsDate( strDate ) Then
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		dtmDate = CDate( strDate )
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	Else
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		Syntax
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	End If
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	On Error Goto 0
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End If
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Set objWMIService = GetObject( "winmgmts://./root/cimv2" )

Set colItems = objWMIService.ExecQuery( "SELECT * FROM Win32_TimeZone", , 48 )

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For Each objItem in colItems
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	dtmGMT = DateAdd( "n", - objItem.Bias, dtmDate )

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	If isDST( dtmDate ) Then
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		dtmGMT = DateAdd( "n", objItem.DaylightBias, dtmGMT )

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	End If
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Next
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Set colItems = Nothing
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Set objWMIService = Nothing
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WScript.Echo dtmGMT & " GMT"
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Function isDST( myDate )
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' Returns TRUE if the specified date is in daylight Saving Time, or FALSE if not.

' The function ignores the 'ambiguous hour' right after the Standard Time transition.

	Dim myDay, myHour, myMinute, myMonth, myYear

	Dim intDaylight, intLocDate, intStandard

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	Dim colItems, objItem, objWMIService
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	' Parse the specified date/time
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	myDate   = CDate( myDate )
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	myDay    = DatePart( "d",    myDate )
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	myMonth  = DatePart( "m",    myDate )
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	myYear   = DatePart( "yyyy", myDate )
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	myHour   = DatePart( "h",    myDate )
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	myMinute = DatePart( "n",    myDate )
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	Set objWMIService = GetObject( "winmgmts://./root/cimv2" )

	Set colItems = objWMIService.ExecQuery( "SELECT * FROM Win32_TimeZone", , 48 )

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	For Each objItem in colItems
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		With objItem
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			' Convert the Daylight Start date/time, specified date/time and Standard Start date/time to long integers (YYYYMMDDHHmm)

			intDaylight = .DaylightMinute + 100 * .DaylightHour + 10000 * LastDoW( .DaylightDayOfWeek, .DaylightMonth, myYear, .DaylightDay ) + 1000000 * .DaylightMonth + 100000000 * myYear

			intLocDate  =  myMinute       + 100 *  myHour       + 10000 * myDay                                                               + 1000000 *  myMonth       + 100000000 * myYear

			intStandard = .StandardMinute + 100 * .StandardHour + 10000 * LastDoW( .StandardDayOfWeek, .StandardMonth, myYear, .StandardDay ) + 1000000 * .StandardMonth + 100000000 * myYear

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		End With
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	Next
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	Set colItems = Nothing
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	Set objWMIService = Nothing
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	' Now that we have the long integers for the date/times, the actual comparison is quite straightforward

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	If intLocDate < intDaylight Then
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		isDST = False
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	ElseIf intLocDate >= intStandard Then
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		isDST = False
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	Else
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		isDST = True
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	End If
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End Function
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Function LastDoW( myDoW, myMonth, myYear, myWoM )

' Returns the day of the month for the specified week day, month, year and week of month

' e.g. LastDoW( 0, 3, 2011, 5 ) will return the last (5) Sunday (0) of March (3) 2011, which is 27

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	Dim i, j
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	j = 0
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	LastDoW = 0
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	For i = 1 To 31
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		If myWoM > j Then
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			If IsDate( myYear & "-" & myMonth & "-" & i ) Then

				If DatePart( "w", CDate( myYear & "-" & myMonth & "-" & i ), vbSunday ) = myDoW + 1 Then

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						j = j + 1
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						LastDoW = i
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				End If
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			End If
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		End If
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	Next
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End Function
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Sub Syntax
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	Dim strMsg
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	strMsg = vbCrLf _
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	       & "GMT.vbs,  Version 1.10" _
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	       & vbCrLf _
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	       & "Return the GMT time and date for the current or specified local time and date" _

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	       & vbCrLf & vbCrLf _
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	       & "Usage:  GMT.VBS  [ date time ]" _

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	       & vbCrLf & vbCrLf _
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	       & "Notes:" & vbTab & "If no date/time is specified, the current date/time is assumed." _

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	       & vbCrLf _
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	       & "      " & vbTab & "DST is checked according to the timezone rules for the current" _

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	       & vbCrLf _
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	       & "      " & vbTab & "year, as found in the registry, but the 'ambiguous hour' right" _

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	       & vbCrLf _
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	       & "      " & vbTab & "after the transition to Standard Time is ignored." _

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	       & vbCrLf & vbCrLf _
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	       & "Written by Rob van der Woude" _

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	       & vbCrLf _
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	       & "http://www.robvanderwoude.com"

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	WScript.Echo strMsg
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	WScript.Quit 1
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End Sub
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