Because the premium based on special offer in the Philippines you'll don't see it on the Spotify site, But if you choose to buy via option 2 (Instructions) through the steps you'll make the exact expiration date will shown, If you buy via the "faster" options (1,3) you'll get from us the exact expiry date in a picture.
I added how it's look like (this picture is from other client purchase today):
http://i.imgsafe.org/6c2a23ff05.png.
If you still not convinced you can send me message via Skype so I can help you be satisfied.