The provided phrase is a noun phrase. Its core grammatical component, or head, is the German compound noun "Sonnenuntergang" (sunset). The entire phrase functions as a single unit to name a specific event.
The date "11. September 2025" acts as a postpositive modifier. It provides specific temporal information that qualifies the noun. Grammatically, this date functions as an appositive, meaning it is placed next to the noun to specify which particular instance of a sunset is being discussed. This structure isolates a singular event from all other possible sunsets.
As a noun phrase, its primary function within a text is to act as a subject, object, or topic identifier. For an article, this construction establishes a precise and unambiguous subject: the specific astronomical event of the sun setting on that exact date. It is not an action (verb) or a descriptor (adjective) but the name of a thing/event.
