～期
～き · ～ki · JLPT N2 · noun
age
🌙📅 期 = 其 (that) + 月 (moon): time counted in moons, so a period
月 means moon or month, and 其 adds the sound. As a suffix, ～き marks a stretch of time: 思春期 (ししゅんき) is puberty, 学期 (がっき) is a school term, 初期 (しょき) is an early stage.
期 — 其 (phonetic) + 月 (moon/time)
期 is a phono-semantic compound. The right side 月 (moon) signals meaning, since lunar cycles marked fixed time periods in ancient China. The left side 其 supplies the sound. Together it points to a set time, period, or appointed date — meanings still carried today in words like 期间 and 日期.