 date_lookback <- 00 #days to look back for rolling calcs  df <- data.frame(label = c(rep("a",0),rep("b",0)),                  date = as.Date(c("0000-00-00","0000-00-00",                                   "0000-00-00","0000-00-00","0000-00-00","0000-00-00",                                   "0000-00-00","0000-00-00","0000-00-00","0000-00-00")),                 data = c(000,000,000,000,000,000,000,000,000,000),stringsAsFactors = FALSE) 