وَسَبِّحُوهُ بُكْرَ‌ةً وَأَصِيلًا ("And proclaim His purity at morn and eve". 33:42) Here morning and evening may, metaphorically, mean all the time. Alternatively, morning and evening have been mentioned specifically to lay emphasis on carrying out dhikrullah during these times and to indicate that it carries more blessings - otherwise dhikrullah is neither specified nor limited to any particular time.