03-date.Rmd

# Dates and Times {#datetime}

```{r, include=FALSE}
source("_common.R", local = knitr::knit_global())
```

## Prerequisites

In this chapter, we will introduce the `{lubridate}` package that is designed to
make it easier to work with dates and times in R.

```{r, message=FALSE}
library(lubridate)
```

## Parsing dates and times

You can easily transform dates and times stored as character vectors to `Date`
or `POSIXct` (or date-time) objects in R using `{lubridate}`. All you need to do
is specify the order of the year `y`, month `m`, and the day `d` and
`{lubridate}` will automatically figure out the format.

```{r}
dmy("3/12/1985")
ymd("85/12/03")
mdy("Dec 3rd 1985")
ydm("85-3rd-december")
```

The same functions can also convert numeric vectors.

```{r}
ymd(19851203)
dmy(31285)
```

You can create Date objects with the time component using a `POSIXct` or
date-time object simply by adding an underscore followed by the order of the
hour `h`, minute `m` and second `s`.

```{r}
ymd_h("1985/12/03 21")
ymd_hm("1985/12/03 21:05")
ymd_hms("1985/12/03 21:05:30")
```

You can also specify the time zone by providing inputs to the argument `tz`. You
can find out more about time zones in R by typing `?timezones` into the console.

```{r}
ymd_hms("1985/12/03 21:05:30", tz = "Singapore")
```

Date vs date-time objects that are created using `{lubridate}`. By default,
dates will be created as `Date` objects without the time component. You can
force the creation of a date-time object by including the timezone `tz`
argument.

```{r}
class(ymd("1985/12/03"))
class(ymd("1985/12/03", tz = "Singapore"))
class(ymd_hms("1985/12/03 21:05:30"))
```

## Extracting components

`{lubridate}` provides simple functions that allows you to easily get different
components of a date or date-time object. These functions are especially useful
when analyzing time-series data and when you want to group your data by a
particular time period.

```{r}

my_dt <- ymd_hms("1985/12/03 21:05:30")
year(my_dt)
month(my_dt)
day(my_dt)
hour(my_dt)
minute(my_dt)
second(my_dt)
wday(my_dt, label = TRUE)
```