I hope that code helps: (it is from UDT DateTime)

Public Overrides Property Value() As String
    Get
        Dim D, T As DateTime
        If ctlDate.Value = "" And ctlTime.Value = "" Then Return ""
        If IsDate(ctlDate.Value) And IsDate(ctlTime.Value) Then
            D = DateTime.Parse(ctlDate.Value)
            T = DateTime.Parse(ctlTime.Value) 'ctlTime.value returns non-normalized time (see constructor)
            D = New DateTime(D.Year, D.Month, D.Day, T.Hour, T.Minute, T.Second)
            If convertTimezone AndAlso HttpContext.Current.Request.IsAuthenticated Then
                Dim objUser As UserInfo = UserController.GetCurrentUserInfo
                Dim UserTime As New Entities.Users.UserTime
                D = UserTime.ConvertToServerTime(D, objUser.Profile.TimeZone)
            End If
            Return D.ToString("s")
        Else
            Return ctlDate.Value & " " & ctlTime.Value 'invalid Value
        End If
    End Get

    Set(ByVal Value As String)
        If IsDate(Value) Then
            Dim d As DateTime = DateTime.Parse(Value)
            If convertTimezone AndAlso HttpContext.Current.Request.IsAuthenticated Then
                Dim objUser As UserInfo = UserController.GetCurrentUserInfo
                Dim UserTime As New Entities.Users.UserTime
                d = UserTime.ConvertToUserTime(d, objUser.Profile.TimeZone)
                Value = d.ToString("s")
            End If
        End If
        ctlDate.Value = Value
        ctlTime.Value = Value
    End Set
End Property

The code is pasted from the UDT module (User Defined Table, a simple core modul)

I chose this code because it shows some tricks. Look at the get part, here are date and time  in their own textboxes. We parse the text to return the date informatio which is still al localized string and it is from the timezone of the current user. So we convert that time again to Server time. At last, as we store everything in a big text column (the price of offering an universal table) as string. We use therefore the "s" format which looks like 2007-11-09T15:31:22. This string is culture neutral.

Setting is the other way round. We take the string, parse it as datetime and convert it back to user time. Afterwards we use also string.format (not part of my example) to fill the textboxes with a localized date again.

DotnetNuke helps you with the Class Usertime to fullfill the timezone conversions. All other stuff is part of the .Net framework.