VB Forum / Win API / December 2006

These might be useful to yourself Chip, or anyone else reading this thread
later. They're extracted from a bigger timezone class I have here.
tTimeZoneInfo is a local TIME_ZONE_INFORMATION variable representing the
selected timezone.

'=====================
Private Function dLoadChangePoint(tDateTime As SYSTEMTIME) As Date
' Loads a daylight-saving changeover date/time (as encoded in
TIME_ZONE_INFORMATION) into a VB Date/time
Dim dTemp As Date
Dim dFirstWeekday As Long           'Weekday of 1st day in month (Sun=0,
Sat=6)

   With tDateTime
       If .wMonth = 0 Then
           oLocale.Error lErrNoDaylightSaving, TypeName(Me)
       Else
           If .wYear > 0 Then
               dTemp = DateSerial(.wYear, .wMonth, .wDay)
           Else
               Select Case .wDay
               Case 1 To 4:        'Week 1 to week 4
                   ' Calculate the first day in the month, and then
calculate the appropriate day
                   ' that the time zone change will occur at
                   dFirstWeekday = Weekday(DateSerial(Year(Date), .wMonth,
1), vbSunday) - 1
                   dTemp = DateSerial(Year(Date), .wMonth, _
                       ((.wDayOfWeek - dFirstWeekday) + (.wDay - 1) * 7) +
1)

               Case 5:             'Last week in month
                   ' Calculate the month's last day, then work back to the
appropriate weekday
                   dTemp = DateSerial(Year(Date), .wMonth + 1, 0)
                   dTemp = DateAdd("d", (.wDayOfWeek - (Weekday(dTemp,
vbSunday) - 1) - 7) Mod 7, dTemp)
               End Select
           End If
       End If

       dLoadChangePoint = dTemp + TimeSerial(.wHour, .wMinute, .wSecond)
   End With
End Function

Public Property Get DaylightStart() As Date
' Returns the date/time at which the transition from standard time to
daylight-saving
' time occurs, or the VB base date (30/12/1899) if no daylight-saving
defined
   If tTimeZoneInfo.DaylightDate.wMonth <> 0 Then
       DaylightStart = dLoadChangePoint(tTimeZoneInfo.DaylightDate)
   End If
End Property

Public Property Get StandardStart() As Date
' Returns the date/time at which the transition from daylight-saving time to
standard
' time occurs, or the VB base date (30/12/1899) if no daylight-saving
defined
   If tTimeZoneInfo.StandardDate.wMonth <> 0 Then
       StandardStart = dLoadChangePoint(tTimeZoneInfo.StandardDate)
   End If
End Property

Public Function InDayLightSaving(dDateTime As Date) As Boolean
' Determines whether a local time is in "daylight saving" mode or not

   If tTimeZoneInfo.DaylightDate.wMonth <> 0 Then
       InDayLightSaving = (dDateTime >=
dLoadChangePoint(tTimeZoneInfo.DaylightDate) And _
           dDateTime < dLoadChangePoint(tTimeZoneInfo.StandardDate))
   End If
End Function
'=================

The following notes are from some updated MSDN docs, and explain how the
changeover times are encoded...

"To select the correct day in the month, set the wYear member to zero, the
wHour and wMinute members to the transition time, the wDayOfWeek member to
the appropriate weekday, and the wDay member to indicate the occurrence of
the day of the week within the month (1 to 5, where 5 indicates the final
occurrence during the month if that day of the week does not occur 5 times).

Using this notation, specify the 2:00a.m. on the first Sunday in April as
follows: wHour = 2, wMonth = 4, wDayOfWeek = 0, wDay = 1. Specify 2:00a.m.
on the last Thursday in October as follows: wHour = 2, wMonth = 10,
wDayOfWeek = 4, wDay = 5."

   Tony Proctor

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