px <- seq(as.POSIXct('1949-01-01'),
          as.POSIXct('1960-12-01'), by = 'month')

d0 <- data.frame(px, y = AirPassengers)

head(d0)

##           px   y
## 1 1949-01-01 112
## 2 1949-02-01 118
## 3 1949-03-01 132
## 4 1949-04-01 129
## 5 1949-05-01 121
## 6 1949-06-01 135

library('forecast')

## Registered S3 method overwritten by 'quantmod':
##   method            from
##   as.zoo.data.frame zoo

y <- d0$y

s <- mstl(x = msts(y, seasonal.period = 12),
          s.window = 'periodic')

autoplot(s, main = '周期成分：時間変動なし')

s <- mstl(x = msts(y, seasonal.period = 12))

autoplot(s, main = '周期成分：時間変動あり')

月間であるため周期は12。