To extend on @BenCole comment, an interesting summary of different models of time perceptions can be found in this paper. Now these models are in a sense more descriptive than the fundamental biological hypothesis mentioned by caseyr547, so might not be ready to call these "explanations", depending on what you mean by the term.
The models meant to give a precise (functional) form to extend to which time perception decrease as the "stock" of experienced time increases.
A simple model which seem to have attracted some attention initially was that of logarithmic time perception (see http://www.kafalas.com/Logtime.html#LM) :
[perception of change from time $t$ to $t'$] = constant * $[\log(t') - \log(t)]$.
Apparently, it was an attempt to link time perception with some interpretation of the so-called Weber-Fechner law relating the intensity of a perception to the magnitude of some initial stimulus of the same nature (e.g. the perceived increase in the weight of an object depending on the initial weight of the object).
Now researcher seems to have found little evidence of such relation in the case of time perception, see the aforementionned article and http://link.springer.com/article/10.3758/BF03204158#page-1. So other model have been developed. The first paper describes some of these attempts at reaching more accurate models.