One Day was conceived as a pop song, following song form in most regards but expanded in duration. The text is by Ross Baglin, with whom Greenbaum has collaborated on a number of other projects. The jazz-inflected vocal line is supported by a delicate accompaniment of alternately running and syncopated figures in the vibes, and subtle contributions from percussion (crotales, glockenspiel and cymbals). There is an optional harmony vocal line cued into the parts.

The vocal part (suitable for either a female or male singer) falls within a one-and-three-quarter octave range from A-F.