I need to do this because in my Outlook calendar I have a lot of recurring events that follow a lunar calendar. This particular feature is not supported by the online version of Outlook calendar. So if I simply sync my data back from the cloud, my recurring events will fall out on the wrong days next year.

Thus my question: is there a way to backup and restore an OST file?
Actually, I don't even need the mail data, only the calendar data.
I could export to a PST file, but it's hard to do that every time I add or modify a calendar entry.