Hi Rupert,
I don't think there's a good way to do this on the fly. I don't think there's an HTML standard for creating hyperlinks for a calendar events (even if there were, it would have many of the issues that mailto links have - i.e., the user has to have their specific email application specified). People could use a variety of different calendar clients (gmail, outlook, apple, etc).
There are ICS files, which are a standard, but generating a file to download on the fly is a little tricky (typically this would be done with a server script, but we
might be able to create a file via SmartBuilder). But then the user would have to download the ICS file, and then know how to import that file into their calendar (I personally have never used an ICS file).
So I just don't think there's a standard for this sort of thing out there. I've seen online apps where you (the author) specify a day, and it will create a bunch of links for different calendars and/or ICS files that you can copy and paste, but not one that generates them on the fly.
Do you know if all your users are using a specific calendar app (like gmail)? Limiting the scope to one particular type of calendar might make this possible - see
this example.
If you find an example of a site doing this, or a standard that we haven't found, let us know!
Your best bet might be to just calculate the next date, and show that to your users, and suggest they add it to their calendar.
Hope that helps!
- Nav