Wow I wondered that yesterday too, and someone got the answer for me. One suggested that you look for the welcome email, or look for the date you recieved the free gift of credits. I was lucky and happened to wonder it out loud when someone who knew could tell me. I am 95 r/l days thought of- and counting. Silly but knowing makes me feel better.
This made me curious, so I found my invitation email, used the converter link (thanks Lord Bacon-o) and found out my imagination-day was last night! (RL time) Edit: Well, really very early this morning ;-)
20th of Widdershins, year 17
:D