We lived near together on a lane in Ch'ang-kan, Both of us young and happy-hearted (5-6)
This poem begins way back in the speaker's childhood, when she's just a little girl playing with a little boy who lives in the neighborhood. The description of herself and her sweetheart in those childhood days as "young and happy-hearted" gives us a sense of the carefree joy that they both felt as children. By beginning in her childhood days, the speaker frames time as an important theme in the poem.
…At fourteen I became your wife (7)
By referencing her age here, the speaker gives us a sense of the years that have passed from her childhood days, and the changes that have taken place. She's no longer a little kid playing with flowers. She's a wife now.
Your footprints by our door, where I had watched you go, Were hidden, every one of them, under green moss (19-20)
The image of the husband's footsteps overgrown with green moss conveys just how much time has passed since the speaker's hubby has left. It's been months and months, and the growing green moss is a strong visual image that conveys this passage of time.
And the first autumn wind added fallen leaves (22)
By referring to the arrival of the autumn and the changing landscape with its "fallen leaves," the speaker also emphasizes the passage of time. As she waits, nature changes around her, and there's still no sign of her hubby.
And now, in the Eighth-month, yellowing butterflies Hover, two by two, in our west-garden grasses (23-24)
The speaker tells us exactly how much time has gone by since her husband's departure: eight months. That's a long time to be separated from our sweetheart. The speaker here also uses nature imagery to convey the passage of time. Butterflies have started appearing (which suggests that it might be springtime). The fact that they're described appearing "two by two" also highlights the speaker's own loneliness. The butterflies are in pairs, whereas she's separated from her "other half": her hubby—sniff.
And because of all this, my heart is breaking And I fear for my bright cheeks, lest they fade (25-26)
The speaker's cheeks are "fading" because she's sad. But they're also fading because she's growing older as she waits for her husband's return. By describing how her looks might change, the speaker also indicates how much time is passing as she sits there waiting for her sweetheart.