The speaker is talking to his lover, Phyllis, and trying to quiet her worries about whether he’ll stay faithful. He does this by pointing out that the past is already gone — nothing but stored images in memory — and the future hasn’t arrived, so neither really belongs to him. The only thing he actually possesses is the present moment, and as quickly as it arrives, he gives it entirely to her. It’s not a grand promise about forever, but a sharp, playful way of saying that right now, completely, he is hers.
The mood is at once tender and irreverent. There’s a shrug behind the logic: if he can manage to be true to her for just this one minute, he’s already done everything heaven allows. Instead of a solemn vow, he offers the intensity of complete presence. The poem feels light, quick, and a little sly — a lover’s argument that turns the problem of inconstancy into a celebration of the fleeting now.