它是把字符串的日期转换为R内置的日期类型POSIXlt，例如

> dts = c("2005-10-21 18:47:22","2005-12-24 16:39:58", "2005-10-28 07:30:05 PDT")
> as.POSIXlt(dts)
[1] "2005-10-21 18:47:22" "2005-12-24 16:39:58" "2005-10-28 07:30:05"