In one of his characteristically striking metaphors, Donne compares the deterioration of a relationship to the lengthening shadows cast as a day progresses to its inevitable conclusion.
Using the well-worn imagery of light and dark, the speaker laments that his relationship with his beloved has declined from its high noon to the time of day when shadows start to lengthen.
When the sun was shining brightly at noon, there were no shadows to speak of. In other words, there was no discord, no disagreements between the two lovers. But as their relationship has matured, they've started to encounter long, dark shadows, which represent the various conflicts that have arisen between them.
Over the course of the day, which represents the duration of the relationship between the two lovers, the shadows, their mutual antagonisms, have grown longer. As a consequence, things are not how they used to be.
Once upon a time, it wouldn't have mattered that their love had not yet attained the highest degree. But things are different now. The couple have now reached a stage in their relationship where they're acutely aware of the challenges that discord and disagreement present to their continuing love. They realize all too well that "love's day is short" and that the high noon of love never lasts for very long.