Spoilers
Makoto is just a girl who might not have put to much thought into her future or how she uses her time. But presently (ha), she has a friend in Chiaki (Red Haired and kind of a slacker) and in Kosuke (dark skinned w/ glasses and studious). Makoto's attitude towards her time and her future experiences a bit of a shift when she, through curiosity and accident, encounters what turns out to be a small time-travel-power-granting device. In no time (heh), she finds herself taking advantage of her new time travel powers, continually leaping to make life a bit more convenient. If she didn't think she needed a better relationship with time, her newfound powers just enabled her to ignore that issue, and for a while the consequences of her actions... for now (heh).
However, while Makoto might not have been thinking to much about her future, her friends and those around her have. Chiaki may actually want to be a bit more than a friend, and Kosuke may be interested in people outside the group. The little circle of friends Makoto may have expected to be there forever, might not actually be forever.
So, Makoto has three interconnected issues to deal with at once: Her relationship to time, Her relationship to consequences, and her relationship to her friends. The naked and main theme of the film, and the idea that Makoto herself has to become acquainted with (through she may hypocritically repeat it elsewhere), is that "Time Waits For No One."