month and ten days; ten months; short time
Classical Chinese-derived expression. Can mean either a period of one month and ten days, ten months, or a relatively short span of time, depending on context. In modern Japanese, it is mostly encountered in historical or literary texts.
In classical texts, 旬月 is sometimes used to mean one month and ten days, or ten months.
The expression 旬月の間 can also refer to a relatively short period of time.