Label month, week, or quarter of a date vector using base R functions

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Use base R functions to label a date vector

Suppose you want to label a date vector with their relevant month, day of week, quarter, or days since a certain date (for example deadlines), how will you go about doing this?

from this:

[1] "1950-11-12" "1980-04-30"
[3] "2010-08-22" "2029-07-01"

to this:

  Q4|Sun|Nov   Q2|Wed|Apr   Q3|Sun|Aug 
"1950-11-12" "1980-04-30" "2010-08-22" 
  Q3|Sun|Jul 
"2029-07-01" 

or just day of week:

      Sunday    Wednesday       Sunday 
"1950-11-12" "1980-04-30" "2010-08-22" 
      Sunday 
"2029-07-01" 

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First create a date vector a1:

a1 <- c("1969-09-26", "1970-05-08")

Check it out:

a1

[1] "1969-09-26" "1970-05-08"

label the vector

we can create vector v1 to name vector a01:

a01 <- a1
v1 <- c("the start", "the end")
names(a01) <- v1
a01

   the start      the end 
"1969-09-26" "1970-05-08" 

or create a separate vector:

a0 <- a1
names(a0) <- c("birthdate", 007)
a0

   birthdate            7 
"1969-09-26" "1970-05-08" 

what happens if we name it with a vector of a different length?

a02 <- a1
names(a02) <- c(5:7) 

Error in names(a02) <- c(5:7) : 
  'names' attribute [3] must be the same length as the vector [2]

you get told off! *** ## using dates let’s try to name this using months!

names(a1) <- months(a1)

Error in UseMethod("months") : 
  no applicable method for 'months' applied to an object of class "character"

seems that we didn’t actually create a date vector! Double check:

class(a1)

[1] "character"

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convert vector to date

a2 <- as.Date(a1)

print & check class:

a2

[1] "1969-09-26" "1970-05-08"

class(a2)

[1] "Date"

Now that we actually have a date vector, we can name the items! ## Labeling by date {.tabset}

label by month

names(a2) <- months(a2)
a2

   September          May 
"1969-09-26" "1970-05-08" 

label by week

a3 <- a2
names(a3) <- weekdays(a3)
a3

      Friday       Friday 
"1969-09-26" "1970-05-08" 

label by year?

a4 <- a2
names(a4) <- years(a4)

Error in years(a4) : could not find function "years"

We get an error, as there is no year function in R base function for time (POSIXt or Date Object). We will get to this later. For now, let’s focus on what we do have: weekdays, months, quarters, & julian (returns numbers of days since the origin).

label by quarters

a5 <- a2
names(a5) <- quarters(a5)
a5

          Q3           Q2 
"1969-09-26" "1970-05-08" 

“julian” - calculate days since

a6 <- a2
names(a6) <- julian(a6)
a6

         -97          127 
"1969-09-26" "1970-05-08" 

Seems that this is calculating days from a date between these two dates. What is going on here? Apparently it’s calculated based on an origin date.

what is the origin date here?

?as.POSIXct

typing the above gives R documentation for a base function, which notes that the origin date in R is ‘1970-01-01 00:00.00 UTC’. ref

Can we specify the origin date?

a7 <- a2
names(a7) <- julian(a7, origin = as.Date("1900-01-01"))
a7

       25470        25694 
"1969-09-26" "1970-05-08" 

note that we cannot change the default origin, but must specify when using. Let’s see what happens when we don’t specify:

a8 <- a7
names(a8) <- julian(a8)
a8

         -97          127 
"1969-09-26" "1970-05-08" 

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All together now

a9 <- a2
names(a9) <- paste(months(a9),weekdays(a9), quarters(a9), sep = "/") #default separator is blank space
a9

September/Friday/Q3 
       "1969-09-26" 
      May/Friday/Q2 
       "1970-05-08" 

This seems rather long, can we abbreviate it?

a10 <- a2
names(a10) <- paste(months(a9, abbreviate = TRUE), weekdays(a9, abbreviate = TRUE), quarters(a9), sep = "-") #as default abbreviate is set to FALSE
a10

  Sep-Fri-Q3   May-Fri-Q2 
"1969-09-26" "1970-05-08" 

I didn’t abbreviate quarters as it can’t get shorter. Note that I just reused a9. Using a vector for labelling does not change the vector itself, as you can see below:

a9

September/Friday/Q3 
       "1969-09-26" 
      May/Friday/Q2 
       "1970-05-08" 

if I names(a9) anew, the new name will overwrite previous name.

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Label with year

To extract the year as a label, we can first extract the year using format(), & then label the vector.

a11 <- a2
v11 <- format(a2, format="%Y")

names(a11) <- v11
a11

        1969         1970 
"1969-09-26" "1970-05-08" 

Et voila. A small exercise.
What else can you do with this?