The Maharal explains the use of the term 'Yashir' - 'he will sing'. Although Moshe actually sang long ago, the Torah often uses future tense when describing an action that takes place over an expanse of time. That is, since the incident is not a split-second occurrence, but rather, one that extends from past to future, the reference to the future is an appropriate one. Az' refers to one particular moment, a freeze-frame in time, removed from the continuum of time and space. Each moment invites the next, time being an extended process with each millisecond only a small part of a larger unit.
Time flows from past to future with the precise present impossible to grasp.
If measured more carefully, we would count by the hundredth of a second. Even that would not be exact, for a finer instrument could measure to the thousandth of a second, and so on, ad infinitum. To be honest then, we must say that time is merely society's measure of a period that stretches from past to future, with the present hidden somewhere between those two points. Much as Hashem stands alone, above the seven days of creation, the Song of the Sea unifies all of time under the aegis of One G-d, acknowledgment that the world rests upon the timeless framework of a supernatural realm. It is for this reason that the Mitzva of Bris Milah is performed on the eighth day, connecting the immortal Jew to a world beyond time.