昔

Bottom sun radical - the indexing component. With the wavy 'days of flood' stacked above, the bottom 日 is the day-counter: many days passed under the catastrophe, hence 'former times.' The radical assignment puts 昔 in the time-measurement family with 时 time, 旧 old, 早 early, 晚 late.

Top 廿 - two 十 (ten) shapes joined at the base, the character for 'twenty.' In 昔 it's the modern stylized shape of an older drawing: rippling waves of floodwater stacked above the sun. The character originally pictured a great flood that 'covered the sun for many days,' giving 昔 its meaning of 'long ago, former times.'