7. one day—a day altogether unique, different from all others [Maurer]. Compare "one," that is, unique (So 6:9; Jer 30:7). Not as Henderson explains, "One continuous day, without night" (Re 22:5; 21:25); the millennial period (Re 20:3-7).
known to … Lord—This truth restrains man's curiosity and teaches us to wait the Lord's own time (Mt 24:36).
not day, nor night—answering to "not … clear nor … dark" (Zec 14:6); not altogether daylight, yet not the darkness of night.
at evening … shall be light—Towards the close of this twilight-like time of calamity, "light" shall spring up (Ps 97:11; 112:4; Isa 30:26; 60:19, 20).