It just changed to '2 days' a few minutes ago. I thought it would change to '4 years' but no? Anybody know when it will be 4 years? I really want to get a new pet slot and waiting is killing me Dx

It will change to 4 years on the exact day that you joined. You can check by adding the "extra userinfo" bit to your profile what the exact day was.

Oh, I didn't even know it's listed there. I joined on the 23rd May so I should get it tomorrow (hopefully). Thank you so very much for your help :3

the whole year/month/weeks thing is based on 30 day months, so it's only approximate. The time of day you joined also matters, so if you joined in the evening on fhe 23 May, then you won't be able to get it at breakfast.