Annoying, right? It's pretty much the most basic piece of information that should be on your profile, and it's impossible to find.
Apart from contacting support, you can look up your player profile on third party sites. For instance:
foedb.com won't tell you when you joined, but the timeline feature may refer to an early event, such as an avatar change. Knowing when that happened may help you with an approximate time for when you started playing.
foestats.com provides a list of your guild associations, with dates. If you were quick to join or create a guild, it'll get you pretty close.
Of course, contacting support every time we're wondering about something is the way to go if we want Inno to be make information more available in general. If enough people ask support, sooner or later, it'll be easier to just make it directly accessible to the players.