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Often when you import date and time data into R, values will be imported as strings.

The easiest way to convert strings to dates in R is with the as.Date() function, which uses the following syntax:

as.Date(x, format)

where:

• x: A single string value or a vector of string values.
• format: The format to use for the date. The default is YYYY-MM-DD.

You can use the ?strftime command in R to view a complete list of arguments available to use for the date format, but the most common ones include:

• %d: Day of the month as decimal number (01-31)
• %m: Month as decimal number (01-12)
• %y: Year without century (e.g. 04)
• %Y: Year with century (e.g. 2004)

This tutorial shows several examples of how to use the as.Date() function in practice.

Example 1: Convert a Single String to a Date

The following code shows how to convert a single string value to a date:

#create string value
x <- c("2021-07-24")

#convert string to date
new <- as.Date(x, format="%Y-%m-%d")
new

[1] "2021-07-24"

#check class of new variable
class(new)

[1] "Date"

Example 2: Convert a Vector of Strings to Dates

The following code shows how to convert a vector of strings to dates:

#create vector of strings
x <- c("2021-07-24", "2021-07-26", "2021-07-30")

#convert string to date
new <- as.Date(x, format="%Y-%m-%d")
new

[1] "2021-07-24" "2021-07-26" "2021-07-30"

#check class of new variable
class(new)

[1] "Date"

Example 3: Convert a Data Frame Column to Dates

The following code shows how to convert a data frame column of strings to dates:

#create data frame
df <- data.frame(day = c("2021-07-24", "2021-07-26", "2021-07-30"),
                 sales=c(22, 25, 28),
                 products=c(3, 6, 7))

#view structure of data frame
str(df)

'data.frame':	3 obs. of  3 variables:
 $ day     : Factor w/ 3 levels "2021-07-24","2021-07-26",..: 1 2 3
 $ sales   : num  22 25 28
 $ products: num  3 6 7

#convert day variable to date
df$day <- as.Date(df$day, format="%Y-%m-%d")

#view structure of new data frame
str(df)

'data.frame':	3 obs. of  3 variables:
 $ day     : Date, format: "2021-07-24" "2021-07-26" ...
 $ sales   : num  22 25 28
 $ products: num  3 6 7

Example 4: Convert Multiple Date Frame Columns to Dates

The following code shows how to convert multiple data frame column of strings to dates:

#create data frame
df <- data.frame(start = c("2021-07-24", "2021-07-26", "2021-07-30"),
                 end = c("2021-07-25", "2021-07-28", "2021-08-02"),
                 products=c(3, 6, 7))

#view structure of data frame
str(df)

'data.frame':	3 obs. of  3 variables:
 $ start   : Factor w/ 3 levels "2021-07-24","2021-07-26",..: 1 2 3
 $ end     : Factor w/ 3 levels "2021-07-25","2021-07-28",..: 1 2 3
 $ products: num  3 6 7

#convert start and end variables to date
df[,c('start', 'end')] = lapply(df[,c('start', 'end')],
                                function(x) as.Date(x, format="%Y-%m-%d"))

#view structure of new data frame
str(df)

'data.frame':	3 obs. of  3 variables:
 $ start   : Date, format: "2021-07-24" "2021-07-26" ...
 $ end     : Date, format: "2021-07-25" "2021-07-28" ...
 $ products: num  3 6 7

You can learn more about the lapply() function used in this example here.

Additional Resources

The following tutorials offer additional information on how to work with dates in R:

How to Sort a Data Frame by Date in R
How to Extract Year from Date in R