VBA SystemTime - 30 days - ms-access

Question

I am working on a code from a previous developer. This code has SystemTime set up.
Is there a way to get today date and minus 30 days in this format?
Code Below:
Public Function GetIsoTimestampTest() As String
Dim st As SYSTEMTIME
'Get the local date and time
GetSystemTime st
'Format the result
GetIsoTimestampTest = _
Format$(st.wYear, "0000") & "-" & _
Format$(st.wMonth, "00") & "-" & _
Format$(st.wDay, "00") & "T" & _
Format$(st.wHour, "00") & ":" & _
Format$(st.wMinute, "00") & ":" & _
Format$(st.wSecond, "00") & "Z"
End Function

Answer 1

Build a native date & time, add -30 days, format as a string:
utcInIsoFormat = Format$(DateAdd("d", -30, _
DateSerial(st.wYear, st.wMonth, st.wDay) _
+ TimeSerial(st.wHour, st.wMinute, st.wSecond)), "yyyy-mm-ddThh:nn:ssZ")

Answer 2

SYSTEMTIME appears to be a custom type defined elsewhere in your code. It's not a standard type available in Access VBA. So to use it effectively, you need to find the definition. Also GetSystemTime is also likely a custom function exclusive to your code. Here's a sample definition of a similar type, although it may not be exactly what's implemented in your system: http://custom-designed-databases.com/wordpress/2011/get-milliseconds-or-seconds-from-system-time-with-vba/
That said, System Time would refer to the Windows system time. You also have a native ability in VBA to get time using the Now() function. (https://msdn.microsoft.com/en-us/library/office/gg278671.aspx) This returns a variable with type Date, which is equivalent to a number where the integer represents days and the decimal represents time of day. An example to get 30 days prior to today would be:
Dim lastMonth as Date
Dim formattedDate as String
lastMonth = Now() - 30
formattedDate = Format(lastMonth, "yyyy-mm-ddThh:nn:ssZ")

Answer 3

DateSerial happily accepts a negative day count. Thus:
Public Function IsoDateMinus30() As Date
Dim st As SYSTEMTIME
Dim Result As Date
' Get the local date and time
GetSystemTime st
Result = DateSerial(st.wYear, st.wMonth, st.wDay - 30)
' To include time:
'
' Result = _
' DateSerial(st.wYear, st.wMonth, st.wDay - 30) + _
' TimeSerial(st.wHour, st.wMinute, st.wSecond)
IsoDateMinus30 = Result
End Function