# swirl Lesson 14: Dates and Times

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1: R Programming
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1: Basic Building Blocks      2: Workspace and Files
3: Sequences of Numbers       4: Vectors
5: Missing Values             6: Subsetting Vectors
7: Matrices and Data Frames   8: Logic
9: Functions                 10: lapply and sapply
11: vapply and tapply         12: Looking at Data
13: Simulation                14: Dates and Times
15: Base Graphics
Selection: 14
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| R has a special way of representing dates and times, which can be
| helpful if you're working with data that show how something
| changes over time (i.e. time-series data) or if your data contain
| some other temporal information, like dates of birth.
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|==                                                        |   3%
| Dates are represented by the 'Date' class and times are
| represented by the 'POSIXct' and 'POSIXlt' classes. Internally,
| dates are stored as the number of days since 1970-01-01 and times
| are stored as either the number of seconds since 1970-01-01 (for
| 'POSIXct') or a list of seconds, minutes, hours, etc. (for
| 'POSIXlt').
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|===                                                       |   6%
| Let's start by using d1 <- Sys.Date() to get the current date and
| store it in the variable d1. (That's the letter 'd' and the
| number 1.)
> d1 <- Sys.Date()
| Your dedication is inspiring!
|=====                                                     |   8%
| Use the class() function to confirm d1 is a Date object.
> class(d1)
[1] "Date"
| That's a job well done!
|======                                                    |  11%
| We can use the unclass() function to see what d1 looks like
| internally. Try it out.
> unclass(d1)
[1] 16997
| Your dedication is inspiring!
|========                                                  |  14%
| That's the exact number of days since 1970-01-01!
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|==========                                                |  17%
| However, if you print d1 to the console, you'll get today's date
| -- YEAR-MONTH-DAY. Give it a try.
> d1
[1] "2016-07-15"
| You are amazing!
|===========                                               |  19%
| What if we need to reference a date prior to 1970-01-01? Create a
| variable d2 containing as.Date("1969-01-01").
> d2 <- as.Date("1969-01-01")
| You are quite good my friend!
|=============                                             |  22%
| Now use unclass() again to see what d2 looks like internally.
> unclass(d2)
[1] -365
| You are amazing!
|==============                                            |  25%
| As you may have anticipated, you get a negative number. In this
| case, it's -365, since 1969-01-01 is exactly one calendar year
| (i.e. 365 days) BEFORE 1970-01-01.
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|================                                          |  28%
| Now, let's take a look at how R stores times. You can access the
| current date and time using the Sys.time() function with no
| arguments. Do this and store the result in a variable called t1.
> t1<-Sys.time()
| That's the answer I was looking for.
|==================                                        |  31%
| View the contents of t1.
> t1
[1] "2016-07-15 20:26:42 EDT"
| You're the best!
|===================                                       |  33%
| And check the class() of t1.
> class(t1)
[1] "POSIXct" "POSIXt"
| You are doing so well!
|=====================                                     |  36%
| As mentioned earlier, POSIXct is just one of two ways that R
| represents time information. (You can ignore the second value
| above, POSIXt, which just functions as a common language between
| POSIXct and POSIXlt.) Use unclass() to see what t1 looks like
| internally -- the (large) number of seconds since the beginning
| of 1970.
> unclass(t1)
[1] 1468628802
| That's a job well done!
|=======================                                   |  39%
| By default, Sys.time() returns an object of class POSIXct, but we
| can coerce the result to POSIXlt with as.POSIXlt(Sys.time()).
| Give it a try and store the result in t2.
> t2<-as.POSIXlt(Sys.time())
| Nice work!
|========================                                  |  42%
| Check the class of t2.
> class(t2)
[1] "POSIXlt" "POSIXt"
| You're the best!
|==========================                                |  44%
| Now view its contents.
> t2
[1] "2016-07-15 20:28:40 EDT"
| You got it!
|===========================                               |  47%
| The printed format of t2 is identical to that of t1. Now
| unclass() t2 to see how it is different internally.
> unclass(t2)
$sec [1] 40.55773$min
[1] 28
$hour [1] 20$mday
[1] 15
$mon [1] 6$year
[1] 116
$wday [1] 5$yday
[1] 196
$isdst [1] 1$zone
[1] "EDT"
$gmtoff [1] -14400 attr(,"tzone") [1] "" "EST" "EDT" | You're the best! |============================= | 50% | t2, like all POSIXlt objects, is just a list of values that make | up the date and time. Use str(unclass(t2)) to have a more compact | view. > str(unclass(t2)) List of 11$ sec   : num 40.6
$min : int 28$ hour  : int 20
$mday : int 15$ mon   : int 6
$year : int 116$ wday  : int 5
$yday : int 196$ isdst : int 1
$zone : chr "EDT"$ gmtoff: int -14400
- attr(*, "tzone")= chr [1:3] "" "EST" "EDT"
| Excellent work!
|===============================                           |  53%
| If, for example, we want just the minutes from the time stored in
| t2, we can access them with t2$min. Give it a try. > t2$min
[1] 28
| You got it right!
|================================                          |  56%
| Now that we have explored all three types of date and time
| objects, let's look at a few functions that extract useful
| information from any of these objects -- weekdays(), months(),
| and quarters().
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|==================================                        |  58%
| The weekdays() function will return the day of week from any date
| or time object. Try it out on d1, which is the Date object that
| contains today's date.
> weekdays(d1)
[1] "Friday"
| You are amazing!
|===================================                       |  61%
| The months() function also works on any date or time object. Try
| it on t1, which is the POSIXct object that contains the current
| time (well, it was the current time when you created it).
> months(t1)
[1] "July"
| All that practice is paying off!
|=====================================                     |  64%
| The quarters() function returns the quarter of the year (Q1-Q4)
| from any date or time object. Try it on t2, which is the POSIXlt
| object that contains the time at which you created it.
> quarters(t2)
[1] "Q3"
| You are amazing!
|=======================================                   |  67%
| Often, the dates and times in a dataset will be in a format that
| R does not recognize. The strptime() function can be helpful in
| this situation.
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|========================================                  |  69%
| strptime() converts character vectors to POSIXlt. In that sense,
| it is similar to as.POSIXlt(), except that the input doesn't have
| to be in a particular format (YYYY-MM-DD).
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|==========================================                |  72%
| To see how it works, store the following character string in a
| variable called t3: "October 17, 1986 08:24" (with the quotes).
> t3<-"October 17, 1986 08:24"
| You are doing so well!
|============================================              |  75%
| Now, use strptime(t3, "%B %d, %Y %H:%M") to help R convert our
| date/time object to a format that it understands. Assign the
| result to a new variable called t4. (You should pull up the
| documentation for strptime() if you'd like to know more about how
| it works.)
> t4<-strptime(t3, "%B %d, %Y %H:%M")
| You're the best!
|=============================================             |  78%
| Print the contents of t4.
> t4
[1] "1986-10-17 08:24:00 EST"
| Excellent job!
|===============================================           |  81%
| That's the format we've come to expect. Now, let's check its
| class().
> class(t4)
[1] "POSIXlt" "POSIXt"
| You got it!
|================================================          |  83%
| Finally, there are a number of operations that you can perform on
| dates and times, including arithmetic operations (+ and -) and
| comparisons (<, ==, etc.)
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|==================================================        |  86%
| The variable t1 contains the time at which you created it (recall
| you used Sys.time()). Confirm that some time has passed since you
| created t1 by using the 'greater than' operator to compare it to
| the current time: Sys.time() > t1
> Sys.time() > t1
[1] TRUE
| Keep working like that and you'll get there!
|====================================================      |  89%
| So we know that some time has passed, but how much? Try
| subtracting t1 from the current time using Sys.time() - t1. Don't
| forget the parentheses at the end of Sys.time(), since it is a
| function.
> Sys.time()-t1
Time difference of 10.09709 mins
| Excellent job!
|=====================================================     |  92%
| The same line of thinking applies to addition and the other
| comparison operators. If you want more control over the units
| when finding the above difference in times, you can use
| difftime(), which allows you to specify a 'units' parameter.
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|=======================================================   |  94%
| Use difftime(Sys.time(), t1, units = 'days') to find the amount
| of time in DAYS that has passed since you created t1.
> difftime(Sys.time(), t1, units = 'days')
Time difference of 0.007476317 days
| Excellent work!
|========================================================  |  97%
| In this lesson, you learned how to work with dates and times in
| R. While it is important to understand the basics, if you find
| yourself working with dates and times often, you may want to
| check out the lubridate package by Hadley Wickham.
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