Time Zone management

Private Type SYSTEMTIME
wYear As Integer
wMonth As Integer
wDayOfWeek As Integer
wDay As Integer
wHour As Integer
wMinute As Integer
wSecond As Integer
wMilliseconds As Integer
End Type
Private Type TIME_ZONE_INFORMATION
Bias As Long
StandardName(31) As Integer
StandardDate As SYSTEMTIME
StandardBias As Long
DaylightName(31) As Integer
DaylightDate As SYSTEMTIME
DaylightBias As Long
End Type
Private Declare PtrSafe Function GetTimeZoneInformation Lib "kernel32" (lpTimeZoneInformation As TIME_ZONE_INFORMATION) As Long
:
Function ConvertLocalToGMT(dtLocalDate As Date) As Date
Dim lSecsDiff As Long

'Get the GMT time diff
lSecsDiff = GetLocalToGMTDifference()
'Return the time in GMT
ConvertLocalToGMT = DateAdd("s", -lSecsDiff, dtLocalDate)
End Function
Public Function ConvertGMTToLocal(gmtTime As Date) As Date
Dim Differerence As Long

Differerence = GetLocalToGMTDifference()
ConvertGMTToLocal = DateAdd("s", Differerence, gmtTime)
End Function

' :
Public Function GetLocalToGMTDifference() As Long
Const TIME_ZONE_ID_INVALID& = &HFFFFFFFF
Const TIME_ZONE_ID_STANDARD& = 1
Const TIME_ZONE_ID_UNKNOWN& = 0
Const TIME_ZONE_ID_DAYLIGHT& = 2

Dim tTimeZoneInf As TIME_ZONE_INFORMATION
Dim lRet As Long
Dim lDiff As Long

'Get time zone info
lRet = GetTimeZoneInformation(tTimeZoneInf)

'Convert diff to secs
lDiff = -tTimeZoneInf.Bias * 60
GetLocalToGMTDifference = lDiff

'Check if we are in daylight saving time.
If lRet = TIME_ZONE_ID_DAYLIGHT& Then
'In daylight savings, apply the bias
If tTimeZoneInf.DaylightDate.wMonth <> 0 Then
'if tTimeZoneInf.DaylightDate.wMonth = 0 then the daylight
'saving time change doesn't occur
GetLocalToGMTDifference = lDiff - tTimeZoneInf.DaylightBias * 60
End If
End If
End Function