I don't get why this one is mae e'n. I thought it might be ydy e
Questions about where, when, how, why, etc are always followed by mae - they are adverbial in nature.
Why does this day o'r gloch instead of amser?
faint o'r gloch is "at what time" when expecting a clock time as an answer ("at three o'clock", etc., rather than "tomorrow" or "in four hours") -- literally something like "how many of the clock" (or "of the bell").