1) make outlook automatically store xls to folder

update reg key to enable "run script"

enable in trust center settings: APPLY MACRO SECURITY SETTINGS to INSTALLED ADD-INS

create the following vbs script:

Public Sub SaveAttachmentsToDisk(MItem As Outlook.MailItem)
    Dim oAttachment As Outlook.Attachment
    Dim sSaveFolder As String
    sSaveFolder = "Z:\Docs\RootShell\cal_xls\"
    For Each oAttachment In MItem.Attachments
        oAttachment.SaveAsFile sSaveFolder & Format(Now, "yyyymmdd") & ".xlsx"
    Next
End Sub

create rule:

subject contains "calendar" and has attachment move to folder "calendars" and run above macro

2) store the files in this repo somewhere calendar can access.

update the config options in xls2ical.php

3) add url http://secretplace.com/xls2ical.php - boom calendar