How does '一勿' Compound to mean sundial?
勿 as in "not" is originally a logograph of a signaling flag. It is not the same logograph as the rays depicted in 昜 (which is 日 rì over  yáng).

It is not uncommon for Chinese characters to have merged similar logographs into one single abstract unit, in the latter case 旦 over 勿.

Take for example 易 (yì), which is unrelated to 昜, although its modern components are 日 and 勿. It is derived from the logograph of a chameleon.