Sayyiduna Abdullah (may Allah be pleased with him) says: What is meant by the "duluk" of the sun is its setting. When the sun sets, the Arabs say of it: "dalakat".
The chain of this report is hasan (good).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 1741