Let I have some data against time :
time <- seq(ISOdate(2007,7,1,0), ISOdate(2008,4,5,23), by = "1 hour")
y <- rnorm(n = length(time))
dat <- data.frame(time = time, y = y)
Now I am trying to create another variable `day_index` which will take value 1 for 2007-07-01...