In my post on 'Conscious Experience and the Mystery of Time', I discussed the fact that the equations of our major physical theories are time-symmetric. There is no explanation for the one-directional and irreversible aspects of time in our experience. Of course there is one well-known theory within science which does incorporate an 'arrow of time': the second law of thermodynamics.
My paraphrase of the second law is that in a closed system, the dispersion of the energy within the system will increase in time. Of course, the constant flow of new energy into open systems can keep energy ordered and localized or foster an increase in order.
One of the interesting sub-plots of modern physics is to see whether a place for the arrow of time can be found as theorists work to integrate and surpass the twin monuments of the twentieth century: general relativity and quantum mechanics.