stroke <- read.csv2("stroke.csv", na.strings=".")
names(stroke) <- tolower(names(stroke))
head(stroke)
stroke <- within(stroke,{
   died <- as.Date(died, format="%d.%m.%Y")
   dstr <- as.Date(dstr, format="%d.%m.%Y")
   end <- pmin(died,  as.Date("1996-1-1"), na.rm = T)
   dead <- !is.na(died) & died < as.Date("1996-1-1")
   obstime <- as.numeric(end - dstr, units="days")/365.25
})