タン, ダン, あき.らか, あきら, ただし, あさ, あした
daybreak, dawn, morning
JLPT1 G7 S5 F5000
日 sun/day + a line# representing the horizon (compare 天) → the sun rising upon the long, flat line of the horizon → *sunrise*; *dawn*. As it was in 天, the line was the phonosemantic element in 旦.
[Full Kanji Etymology List]