This feels like a really big missing feature in this game. I know they had the companion app for a bit, but that application is basically dead for a number of reasons (not everyone uses a smart phone, many people use the security dongle and not the phone app for two factor authentication, many people play on console applications, etc). Using third party tools such as Google Calendar has it's limits, as it requires people to use their real email or some out of game identity to do the sign up. This makes it difficult to track who really is coming or not until everyone is familiar with one another.
Just about every other MMO I've been playing has some kind of event calendar feature, whether by third party mods or through the game itself. It would definitely help with the organization of events more than having to paste a bunch of dates.
Event calendars provide players with the following features:
1) Event posting and sign up. Usually with a "Maybe", "not interested", and "Attending".
2) Shows in-game yearly events like the Anniversary of FFXIV, the Starlight Festival, etc, and indicates when the event ends.