Access 2010 - Last Sunday in March & October - ms-access-2010

Question

I have a series of dates, which are recorded in both BST and GMT (depending upon the time of year). I want them to all be in GMT but I can't work out how to get Access to return the last Sunday in March (when we switch from GMT to BST) and the last Sunday in October (when we switch back).
All help appreciated!
I'm working in Access 2010.
Thank you in advance.

Answer 1

You can use this generic function:
' Calculates the date of the occurrence of Weekday in the month of DateInMonth.
'
' If Occurrence is 0 or negative, the first occurrence of Weekday in the month is assumed.
' If Occurrence is 5 or larger, the last occurrence of Weekday in the month is assumed.
'
' If Weekday is invalid or not specified, the weekday of DateInMonth is used.
'
' 2016-06-09. Gustav Brock, Cactus Data ApS, CPH.
'
Public Function DateWeekdayInMonth( _
ByVal DateInMonth As Date, _
Optional ByVal Occurrence As Integer, _
Optional ByVal Weekday As VbDayOfWeek = -1) _
As Date
Const DaysInWeek As Integer = 7
Dim Offset As Integer
Dim Month As Integer
Dim Year As Integer
Dim ResultDate As Date
' Validate Weekday.
Select Case Weekday
Case _
vbMonday, _
vbTuesday, _
vbWednesday, _
vbThursday, _
vbFriday, _
vbSaturday, _
vbSunday
Case Else
' Zero, none or invalid value for VbDayOfWeek.
Weekday = VBA.Weekday(DateInMonth)
End Select
' Validate Occurence.
If Occurrence <= 0 Then
Occurrence = 1
ElseIf Occurrence > 5 Then
Occurrence = 5
End If
' Start date.
Month = VBA.Month(DateInMonth)
Year = VBA.Year(DateInMonth)
ResultDate = DateSerial(Year, Month, 1)
' Find offset of Weekday from first day of month.
Offset = DaysInWeek * (Occurrence - 1) + (Weekday - VBA.Weekday(ResultDate) + DaysInWeek) Mod DaysInWeek
' Calculate result date.
ResultDate = DateAdd("d", Offset, ResultDate)
If Occurrence = 5 Then
' The latest occurrency of Weekday is requested.
' Check if there really is a fifth occurrence of Weekday in this month.
If VBA.Month(ResultDate) <> Month Then
' There are only four occurrencies of Weekday in this month.
' Return the fourth as the latest.
ResultDate = DateAdd("d", -DaysInWeek, ResultDate)
End If
End If
DateWeekdayInMonth = ResultDate
End Function
Then:
LastSundayMarch = DateWeekdayInMonth(DateSerial(Year(SomeDateOfYear), 3, 1), 5, vbSunday)
LastSundayOctober = DateWeekdayInMonth(DateSerial(Year(SomeDateOfYear), 10, 1), 5, vbSunday)
or as functions:
Public Function DateLastSundayMarch(ByVal DateOfYear As Date) As Date
DateLastSundayMarch = DateWeekdayInMonth(DateSerial(Year(DateOfYear), 3, 1), 5, vbSunday)
End Function
Public Function DateLastSundayOctober(ByVal DateOfYear As Date) As Date
DateLastSundayOctober = DateWeekdayInMonth(DateSerial(Year(DateOfYear), 10, 1), 5, vbSunday)
End Function
Now you can have expressions like:
=DateLastSundayOctober([SomeDateField])
to be used as ControlSource for a control or in the GUI query designer.

Answer 2

This will work:
Option Explicit
Function getLastSundayOfMarchOfThisYear() As Date
getLastSundayOfMarchOfThisYear = getLastSundayOfMonthOfThisYear(3)
End Function
Function getLastSundayOfOctoberOfThisYear() As Date
getLastSundayOfOctoberOfThisYear = getLastSundayOfMonthOfThisYear(10)
End Function
Private Function getLastSundayOfMonthOfThisYear(month As Long) As Date
Debug.Assert month >= 1 And month <= 12
Dim i As Long, _
tmpDate As Date, _
daysInMonth As Long
daysInMonth = Day(DateSerial(year(Date), month + 1, 1) - 1)
For i = daysInMonth To 1 Step -1
tmpDate = CDate(year(Date) & "-" & month & "-" & i)
If Weekday(tmpDate) = vbSunday Then
getLastSundayOfMonthOfThisYear = tmpDate
Exit Function
End If
Next
End Function