Formatting date/time

In data analysis, it is common to encounter date and time data types. Perhaps, the simplest functions related with date are Sys.Date(), which returns the current date, and Sys.time(), which returns the current time.

As the book is being rendered, the date is printed as follows:

Sys.Date()
## [1] "2016-02-26"

And the time is:

Sys.time()
## [1] "2016-02-26 22:12:25 CST"

From the output, the date and time look like character vectors, but actually they are not:

current_date <- Sys.Date()
as.numeric(current_date)
## [1] 16857
current_time <- Sys.time()
as.numeric(current_time)
## [1] 1456495945

They are, in essence, numeric values relative to an origin and have special methods to do date/time calculations. For a date, its numeric value means the number of days passed after 1970-01-01. For a time, its numeric value means the number of seconds passed after 1970-01-01 00:00.00 UTC.

Parsing text as date/time

We can create a date relative to a customized...