16.1. Date Time

Current date and time:

> date()
[1] "Tue Oct 31 12:43:00 2017"

This is a character object.

Current date:

> Sys.Date()
[1] "2017-10-31"

This is a Date object.

Current time:

> Sys.time()
[1] "2017-10-31 12:44:11 IST"

Current time is a POSIXct object.

Following options are available in R for date time manipulation:

• Built-in as.Date function supports dates (but no time)
• chron library supports date and time but no time-zones
• POSIXct and POSIXlt classes provide support for time zone too.

Using asDate function:

> as.Date('1982-03-18')
[1] "1982-03-18"
> as.Date('1982/03/18')
[1] "1982-03-18"
> d <- as.Date('1982/03/18')
> typeof(d)
[1] "double"
> class(d)
[1] "Date"
> mode(d)
[1] "numeric"

The default format is YYYY-MM-DD where the separator can be changed.

Specifying a custom format:

> as.Date('18/03/1982', format='%d/%m/%Y')
[1] "1982-03-18"

The codes for the date format are listed in the table below.

Code	Value
%d	Day of the month (decimal number)
%m	Month (decimal number)
%b	Month (abbreviated)
%B	Month (full name)
%y	Year (2 digit)
%Y	Year (4 digit)

Number of dates since Jan 1, 1970:

> as.numeric(as.Date('1982/3/18'))
[1] 4459

Date arithmetic:

> d <- as.Date('1982/03/18')
> d
[1] "1982-03-18"
> d + 1
[1] "1982-03-19"
> d + 30
[1] "1982-04-17"
> d - 1
[1] "1982-03-17"
> d - 30
[1] "1982-02-16"

Preparing a list of dates:

> d + seq(1, 10, by=3)
[1] "1982-03-19" "1982-03-22" "1982-03-25" "1982-03-28"
> d2 <- d + seq(1, 10, by=3)

Identifying corresponding weekdays:

> weekdays(d2)
[1] "Friday"   "Monday"   "Thursday" "Sunday"

Corresponding months:

> months(d2)
[1] "March" "March" "March" "March"

Corresponding quarters:

> quarters(d2)
[1] "Q1" "Q1" "Q1" "Q1"