one period
朞 centers on a fixed cycle or time unit: one complete period, such as a full year or a round of time.
朞 is a variant form of 期, combining 其 (likely phonetic) with 月 (moon/month) to express a period of time. The exact historical development is uncertain.
The top 其 looks like a container, and the bottom 月 is the moon. Picture the moon completing one full cycle inside a marked container: that is one period.
For キ, imagine a key that unlocks a new period of time: key -> キ, and the moon marks the start of a new cycle.