I can't seem to find anything related to Date within will_paginate itself, which seems great; but fwiw Date.today's behavior seems to be changed once I required will_paginate (2.3.11). See : http://pastie.org/553050
I am under-caffeinated atm, though. So this may be completely unrelated. ^^;
Hey. The mainstream version of will_paginate requires ActiveSupport, which probably extends Date in this way.
To use will_paginate with Sinatra and in the most unobtrusive way, use the "agnostic" branch from github (you can easily build the 3.0 gem with rake tasks). This version does not have any dependencies, not even ActiveSupport.