i find that too.. in any situation- not just studying.

not looking at the clock makes it less stressfull and it seems to go quicker

i would advise just doing little bits at a time- break it up, and make sure you give yourself enough time before the exam or what ever, so you're not rushed

don't for example think at the last moment before a test- i've not done this, this, this or this... because then that will make you stressed