Date classes

Formatting dates

Storing dates

POSIXct is the number of seconds since 12:00 am on January 1st, 1970 (UTC), also known as “unix time.” While it may seem ludicrous to represent time this way (as one giant number), this actually has a lot of advantages. You don’t have to worry about time zone conversions, and having the time as just a number makes it a lot easier for machines to handle. For human readability, you have to run conversions for day, month, year, and time.

as.POSIXct(Sys.Date())

## [1] "2020-04-22 17:00:00 PDT"

POSIXlt is a more human-readable form of the time. It is a named list of vectors that includes all of the information you could want about a date.

plt <- as.POSIXlt(Sys.Date())
objects(plt)

## [1] "hour"  "isdst" "mday"  "min"   "mon"   "sec"   "wday"  "yday"  "year"

Generally, it’s a good idea to store dates or timestamps as POSIXct objects. This ensures uniformity, and means that you can operate on all the dates without worrying about differences in how they’re stored (month-day-year versus day-month-year, for example). You can always convert these objects to another form if that’s what you need.

Manipulating dates

Dates are just numbers, meaning you can do arithmetic operations with them.

You can add and subtract time:

You can compare times with logicals:

Plotting with dates

They also play well with ggplot, and there are convenient operations you can do specifically on POSIX objects to customize things.