表 fused fur 毛 and clothing 衣: ancient coats wore the fur outward, so 表 meant the outer surface. What shows on the outside expresses what's within — and a watch (手表) shows time on its face.
手表 wristwatch, 表示 to express, 表演 to perform, 课程表 the class schedule. Cousins on your mother's side are 表哥、表妹 — 'outside' cousins, versus 堂 cousins who share your surname. Kinship, mapped by a coat.
Fur 毛 worn outside the coat 衣 — the SURFACE that EXPRESSES; time shown on a WATCH face.
表白 'to express whiteness' means confessing your love — laying your surface bare. Chinese students agonize over 表白 timing the way others agonize over exams.