$15 for rock-hard resin

I recently opened up my purchase of UV resin from December 07 and found out the entire bottle already hardened. I used only about 4oz out of 15, and I was wondering if this is supposed to happen? I kept the bottle in a room which doesn't receive much sunlight and I'm not sure if it just expired, or the sunlight affected it somehow. Anyone have troubles with resin hardening before they're able to use it all? 
