是这样吗？

library(lubridate)
months_since_1960 <- c(0, 1, 12, 13, 24, 25)
date_ref = as.Date("1960-01-01")
date_ref %m+% months(months_since_1960)

1）yearmon首先创建一个yearmon变量，它在内部用年份+分数来表示日期，其中12个月的分数为0，1/12，...，11/12。然后我们可以使用它或转换为Date，给出几种可能性。

library(zoo)
months_since_1960 <- c(0, 1, 12, 13, 24, 25)

ym <- as.yearmon(1960) + months_since_1960 / 12

ym # year and month
## [1] "Jan 1960" "Feb 1960" "Jan 1961" "Feb 1961" "Jan 1962" "Feb 1962"

as.Date(ym) # first of month
## [1] "1960-01-01" "1960-02-01" "1961-01-01" "1961-02-01" "1962-01-01" "1962-02-01"

as.Date(ym, frac = 1) # last of month
## [1] "1960-01-31" "1960-02-29" "1961-01-31" "1961-02-28" "1962-01-31" "1962-02-28"

2）Base R使用Base R（无软件包），我们可以这样做以获取每月的第一天：

dat <- months_since_1960 |>
  lapply(\(x) tail(seq(as.Date("1960-01-01"), length = x+1, by = "month"), 1)) |>
  do.call(what = "c")
dat
## [1] "1960-01-01" "1960-02-01" "1961-01-01" "1961-02-01" "1962-01-01" "1962-02-01"

或上月末

as.Date(cut(dat + 31, "month")) - 1
## [1] "1960-01-31" "1960-02-29" "1961-01-31" "1961-02-28" "1962-01-31" "1962-02-28"

**2a）**或者，如果需要，我们可以使用相同的转换来获得月末：

dat2 <- as.Date(ISOdate(1960 + (months_since_1960 %/% 12), months_since_1960 %% 12 + 1, 1))
dat2 
## [1] "1960-01-01" "1960-02-01" "1961-01-01" "1961-02-01" "1962-01-01" "1962-02-01"