Besides clocks and the position of the sun, change may be the best way to measure time. (In fact, since we can use clocks and the position of the sun to tell time only because they change, we could say that change is the only way to measure time.) When things change, we know that time has passed. Since The Time Machine tells a story about a great deal of time passing (who doesn't want to visit the year 802,701?), there's also a great deal of change in the story – change to the environment (the future is hotter…until the sun starts dying), change to social structure, change to the species. Everything changes in The Time Machine, except maybe Time itself and the other forces of nature – those seem to be pretty constant.
While The Time Machine shows that everything is temporary, the moral is that it's still important to make choices and act.