What to people use for calendar invites?

[dm-list-email-notmuch at scs.stanford.edu]

I've been running into this issue lately where I agree to meet people
and we say it's confirmed, but if don't send them a calendar invite of
mime type text/calendar, then it's as if we never agreed and they don't
show up.  I get, "Oh, you never sent me a calendar invite so it wasn't
in my calendar."

I'm wondering if others have this problem and have figured the easiest
way to integrate notmuch with some kind of calendaring software that
generates ics or text/calendar attachments.

Thanks,
David