I have never heard of this issue with the Sensation, but here are my tips, that I would try if the problem occured to me.
1) Are you rooted, and running a custom ROM? If you are, try flashing another ROM, or, contact the developer of the ROM, and see if anyone else has had this problem.
2) Are you set to 24 hour time, when you want 12 hour time, or the other way around? To check: Go to Settings >> Date and Time >> Check/Uncheck Use 24/Hour format.
3) Are you using an auto app killer? Applications that automatically kill apps to the times you set it, such as whenever you wake your phone, or every few hours, delete the app! You think it may save your power, but it does not, the sensation is fine without it, plus those apps are known to slowly take the full capacity charge out of batteries.
4) Try keeping it off Automatic, and see how it goes.
If this problem keeps up contact HTC. They may provide you will further instructions on how to solve your issue, and with the Sensation, I hear sometimes they may just ship you a new sensation free of charge. Good luck!