A past note is experienced as different than a present note not because of any difference of content— both are the selfsame C— but because it is given in a different form. The previous note has undergone a modification in its mode of presentation, in the manner of its being experienced, while remaining the same note and it is this modification which indicates that it is past. It is modified as "sinking away," as the "having-just-been."
As an analogy, imagine if I look at the table and then put on pink sunglasses and continue to look at the table. I do not now see two things, a table and pink glasses; I see the self-same table as before, just coloured pink. Similarly the retained note is the self-same note modified by a different "colour," the colour of the just past.