Lines <- "1/19/1990 1.22
1/20/1990 1.25
1/23/1990 1.26
1/24/1990 1.26
1/25/1990 1.26
1/26/1990 1.26
2/1/1990 1.34
2/2/1990 1.36
2/5/1990 1.22
2/6/1990 1.22
2/7/1990 1.22
2/8/1990 1.22"

DF <- read.table(text = Lines, col.names = c("Date", "Value"))
DF$Date <- as.Date(DF$Date, "%m/%d/%Y")
aggregate(DF$Value, list(ym = format(DF$Date, "%Y-%m")),
   function(x) if (length(x) >= 5) x[5] - x[2] else NA)

library(zoo)
library(chron)
read.zoo(text = Lines, FUN = chron, FUN2 = as.yearmon,
  aggregate =  function(x) if (length(x) >= 5) x[5] - x[2] else NA)

R>        from <- as.Date("2009-04-07")
R>        to <-as.Date("2009-04-14")
R>        getHolidayList("UnitedStates", from, to)
NULL
R>        to <- as.Date("2009-10-7")
R>        getHolidayList("UnitedStates", from, to)
[1] "2009-05-25" "2009-07-03" "2009-09-07"
R>

R>        from <- as.Date("2009-04-07")
R>        to<-as.Date("2009-04-14")
R>        businessDaysBetween("UnitedStates", from, to)
[1] 5
R>

workdays = function(iniDate, endDate, holidays) {
  theDates = seq(from=iniDate,to=endDate,by="day")
  isHoliday = theDates %in% holidays
  isWeekend = (as.POSIXlt(theDates)$wday) %in% (c(0,6))
  return (sum(!isHoliday & !isWeekend))
}