Fixed Effects and the Limits of Regression
Module 7.3: Working with Dates
All materials can be found at alexcardazzi.github.io.
Dates in R
As one last R tool for this course, we will discuss an important package that does not take much time to learn. Often times, the data we are working with has some time dimension (e.g., yearly, monthly, quarterly, daily, etc.) to it, so learning how to work with dates is helpful. The lubridate package in R is outfitted for this exact purpose, so we are going to learn our way around some of its functions.
Yeah, the name is hilarious. Mount Rushmore of package names.
lubridate
Let’s go through some of the important functions / capabilities in lubridate.
Note: There is a great cheatsheet available online.
ymd(), dmy(), mdy(): when you read data into R and one of the columns has entries like "January 2 2014", we can use these functions to help us out. Note that y stands for year, d stands for day, and m stands for month, so just choose the function that makes sense for your situation.
Did the last one give you a warning? February 30th does not exist, so lubridate returns NA instead of a date.
Output from these functions appear in year-month-day format, but are actually numeric under the hood. We can see this by executing as.numeric(mdy("January 2 2014")): 16072. This is the number of days since January 1, 1970.
Once your data is converted to well-behaved date objects, we can begin to manipulate them. We can extract the year, month, day, etc. using lubridate’s helpfully named year(), month(), and day() functions. We can also grab the weekday by using wday(df$date, label = TRUE).
Next, we can use floor_date(df$date, "quarter") to “round” our dates down to the first day of the quarter, month, year, whatever.
Finally, we can calculate differences in dates by using simple subtraction. However, often times we want to know the number of, say, months between two dates. For this, we can use the following code: interval(date1, date2) %/% months(1). For example, the number of whole months from January 2, 2014 to March 15, 2015 is 14.