A while ago I decided that I wanted my schedule to show up on google calendar so I could look at one calendar that had both my events and julia's events. So I wrote something to convert it into ical format, and set my scripts to run that whenever I changed the calendar. At the time, this turned out not to be very helpful: google would load my calendar feed but then wouldn't make updates. So I left it alone for a year.

Now my situation changes: I have a mac. So I open up ical, import my feed, import julia's google calendar feed, and everything just works [1]. The reason this all worked is that google advertised support of someone else's open format, ical, so that when I wanted google calendar support that's what I implemented. If they had instead chosen to write their own format or something, I would have needed to start all over to get ical integration and might never have taken the time.

[1] Actually, it doesn't. I hadn't implemented all-day events properly, so I had to tweak my feed generation scripts.