Very simply, the song is about the narrator's need to live in the moment. She realizes her mortality is a very real concept, but, much like what blondie is saying is "slow motion", when one is living life to the fullest, time has no meaning. 11:59 is the last minute of the day, but that doesn't matter to the Narrator, becuase she is going to live her life as fully as she can.
Very simply, the song is about the narrator's need to live in the moment. She realizes her mortality is a very real concept, but, much like what blondie is saying is "slow motion", when one is living life to the fullest, time has no meaning. 11:59 is the last minute of the day, but that doesn't matter to the Narrator, becuase she is going to live her life as fully as she can.