Caching of external iCal calendar exports

[stefanw]

I wrote an iCal export plugin for a legacy application, which is supposed to display its events in Zimbra. This works fine for the initial export, but any subsequent changes in the legacy application's data don't show up in the Zimbra calendar view. Looking at the legacy app's logs tells me that Zimbra only requests the iCal file once, and apparently uses a cached value afterwards. How can the cache duration für synchronized external calendar sources be adjusted?

I'm not very familiar with Zimbra, and I'm not an administrator of this particular installation, just a contractor who was asked to look into this issue.

Thanks in advance.

[EDIT] PS: sorry, I forgot to mention this earlier: clicking the "Refresh" button or logging in and out does not cause Zimbra to request a fresh iCal export file.