Get ready for some close reading, dear Shmoopers. We're about to bust out our magnifying glass to show you that "Upon Julia's Clothes," which is, on the whole, about Julia and her clothes (shocker!) is also about a Big Idea we like to call Time. In fact, Herrick is well known for writing about time, and about making the most of our short lives (see, for example, "To the Virgins, to Make Much of Time"). And in the brief span of a poem, Herrick certainly makes the most of his raw materials: the silks and liquefaction of Julia's ensemble.
By using words that mark time, like "whenas," "then," "next," and "when," the speaker shows us that he'll be in love with Julia only so long as she's wearing a gorgeous gown.
This poem's odd preoccupation with time shows that the speaker's affections for Julia are conditional. They're not permanent, lasting, or enduring.