更新：澄清了回答评论者的问题和代码。
我不知道为什么date.ie创建了一个POSIXct对象，但date.ipt创建了一个数字对象。在这两种情况下，我都使用lubridate来添加日期。
set.seed(34859)
date.ie = (ymd("2013/07/01", tz="Africa/Nairobi")) + days(round(runif(10)*(150),
digits=0))
# creates this POSIXct object
# structure(c(1382821200, 1373922000, 1385326800, 1385154000, 1381093200,
# 1381870800, 1372971600, 1377982800, 1382216400, 1383512400), class = c("POSIXct",
# "POSIXt"), tzone = "Africa/Nairobi")
date.ie <- format(date.ie, format="%Y-%m-%d")
# creates this character object
# c("2013-10-27", "2013-07-16", "2013-11-25", "2013-11-23", "2013-10-07",
# "2013-10-16", "2013-07-05", "2013-09-01", "2013-10-20", "2013-11-04"
# )
date.ie2 <- ymd(date.ie)
treat <- rep(0:1, 5)
start.ipt <- (sample(x=c(0,1), size=10, replace=TRUE,
prob=c(.5, .5))) # changed to even prob for small dataset
更新2：
下一个块是我的旧方法，由于ifelse的行为方式，它无法正常工作。谢谢@hadley指出这一点。
date.ipt <- ifelse(treat==0 & start.ipt==1,
date.ie2 + days(round(runif(10)*(90), digits=0)),
ifelse(treat==1 & start.ipt==1,
date.ie2 + days(round(runif(100)*(60), digits=0)),NA))
# creates this numeric object
# c(1384992000, 1375488000, NA, NA, NA, 1385337600, 1375142400,
# 1381449600, 1383782400, 1384560000)
date.ipt = format(data$date.ipt, format="%Y-%m-%d")
这是我的修复。而不是使用ifelse一步创建date.ipt，我在几个中做。我到达了我想去的地方，但需要一段时间。希望评论者能够提出如何更有效地做到这一点的建议。
date.ipt.c <- date.ie2 + days(round(runif(10)*(90), digits=0))
date.ipt.c = format(date.ipt.c, format="%Y-%m-%d")
date.ipt.t <- date.ie2 + days(round(runif(10)*(60), digits=0))
date.ipt.t = format(date.ipt.t, format="%Y-%m-%d")
date.ipt <- ifelse(treat==1 & start.ipt==1, date.ipt.t, ifelse(treat==0 & start.ipt==1, date.ipt.c, NA))
date.ipt <- format(date.ipt, format="%Y-%m-%d")
date.ipt2 <- ymd(date.ipt)
days.to.ipt <- difftime(date.ipt2, date.ie2, tz="Africa/Nairobi", units="days")
data <- data.frame(date.ie, treat, start.ipt, date.ipt, days.to.ipt)