Notice how the code uses Right$ to ensure the milliseconds value is left padded with zeros if necessary. For example if GetLocalTime returns 30 milliseconds, the program displays 030.

(Based on a suggestion from Michael Rosquist).

' Return an ISO 8601 compliant timestamp.
Private Function GetIsoTimestamp() As String
Dim st As SYSTEMTIME

    ' Get the local date and time.
    GetLocalTime st

    ' Format the result.
    GetIsoTimestamp = _
        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") & "." & _
        Right$("000" & Format$(st.wMilliseconds), 3)
End Function