thedate    | weekdays
-----------+--------------------------
2017-11-21 | 0
2017-11-22 | 195
2017-11-23 | 390
2017-11-24 | 585
2017-11-25 | 585 (because it's a saturday)

dates <- as.Date("2017-11-21"):as.Date("2020-11-20")
forecasting <- data.frame(thedate = as.Date(dates, origin = "1970-01-01"))
forecasting$weekdays <- 195 * sum(!weekdays(seq(Sys.Date(), forecasting$thedate, "days")) %in% c("Saturday", "Sunday"))