Recently, while working on quantifying the inpatient workload volume of routine tests as a function of the day of the week, I needed to be able to count the number of Mondays, Tuesdays, etc in a time–interval so I could calculate the average volume for each weekday in a time–interval.

The lubridate package makes this a very easy thing to do. Suppose the first date in your series is 21-May-2015 and the last date is 19-Aug-2015.

library(lubridate)
startDate <- dmy("21-May-2015")
endDate <- dmy("19-Aug-2015")

Now build a sequence between the dates:

myDates <-seq(from = startDate, to = endDate, by = "days")
head(myDates)

## [1] "2015-05-21 UTC" "2015-05-22 UTC" "2015-05-23 UTC" "2015-05-24 UTC"
## [5] "2015-05-25 UTC" "2015-05-26 UTC"

The function wday() tells you which weekday a date corresponds to with Sunday being 1, Monday being 2 etc.

wday(startDate)

## [1] 5

This means that 2015-05-21 was a Thursday. To get the abbreviation, you can enter:

wday(startDate, label = TRUE)

## [1] Thurs
## Levels: Sun < Mon < Tues < Wed < Thurs < Fri < Sat

and to get the full name of the day:

wday(startDate, label = TRUE, abbr = FALSE)

## [1] Thursday
## 7 Levels: Sunday < Monday < Tuesday < Wednesday < Thursday < ... < Saturday

Leap years are accounted for:

wday(dmy("29-Feb-1504"), label = TRUE, abbr = FALSE)

## [1] Monday
## 7 Levels: Sunday < Monday < Tuesday < Wednesday < Thursday < ... < Saturday

So, we can use this as follows to find the Mondays:

which(wday(myDates)==2)

##  [1]  5 12 19 26 33 40 47 54 61 68 75 82 89

So the whole code to count them is:

startDate <- dmy("21-May-2015")
endDate <- dmy("19-Aug-2015")
myDates <-seq(from = startDate, to = endDate, by = "days")
length(which(wday(myDates)==2))

## [1] 13

I was born on August 04, 1971. This was a Wednesday. How many Wednesdays since I was born?

startDate <- dmy("04-Aug-1971")
endDate <- dmy("25-Nov-2015")
myDates <-seq(from = startDate, to = endDate, by = "days")
length(which(wday(myDates, label = TRUE)=="Wed"))

## [1] 2313

Which means, today I am 2312 weeks old today! Hurray. This is not a typo. The time interval is flanked by Wednesdays so there is one more Wednesday than the number of weeks in the interval. I thank my first–year calculus prof for beating this into me with reference to Simpson's Rule numerical integration.

Hope that comes in handy.

-Dan

Teach us to number our days, that we may gain a heart of wisdom. Psalm 90:12.