I want to talk briefly about dates and time in R, which is a very, is a very special topic and could require a lot more time. And I have something I need to talk a little bit about how R represents dates and times, and how you can use them in kind of ara, arithmetic and data analysis types of computations. [NOISE] so R has a special way to represent dates and times. And they're, they're represented using special data classes. So, in the past, we talked about different data types like lists. And character vectors. And, numeric vectors, and so. This is just another type of data on top of those kinds of classes. So dates are represented by the date class. and, times are represented by two separate classes: the POSIXct and the POSIXlt class. So dates are basically. Don't have times attached to them. They represent a day, in a year in a month. And the, kind of, you can figure them in a kind of a year, month, day format so for example, this date is 1970, January 1st and so internally the dates are stored as the number of days since 1970 January 1st. That particular detail is not very important but in case you're wondering you don't know how they, how the, how R actually does calculations based on dates. Times are stored internally as the number of seconds. Since 1970, January 1st. And so, that's, again, another underlying detail. That's not very important, but it maybe useful to know, sometimes. So, the way dates in R, in R work, is you can take a character screen. Like this following 1970-01-01. And convert it into a date, using as.Date function. That's probably the most common way that you'll start your, begin working with dates. And, you'll notice that if you print out this object. You'll get something that looks like a character string. Now it's not actually a character string but it will print out that way because there's a special print method. Now if I unclass the object here you'll see I get the number 0. Remember? Because the dates are stored internally as the number of days since 1970, January 1st and since. 1970 January 1st. 0 days from that point. You'll get 0. If I. If I input January 2, 1970, then you'll see that underline is represented as a number 1, because that's 1 day after 1970 January 1st. If you had a. If you had a date that was before 1970 Then, they'd be represented as negative numbers. so, but that's just for your little background. You don't, you don't have to worry about the underlying representation. Ultimately, what you need to know, is that dates are stored as objects of the date class. Times, on the other hand, are represented as two possible types. One is called POSIXct and the other POSIXlt. So, POSIX is a family of computing standards for how things should be done on certain types of computers or how data should be represented and so there's a there's a family of standards for how to represent dates and times and pos and so POSIX a that's part of the POSIX standard. So [COUGH] in the POSIXct class. Times are represented at just as very large integers. And so it's a useful type of class if you want to say, store times in a data frame or something like because it's basically it's like a big integer vector. POSIXlt stores a time as a list. underlying, and so, and it stores a bunch of other useful information about a given time, for example what's the day of the week of that time, what's the the day of the year, the day of the month, or the month itself. And there are a number of generic functions that operate on both dates and times. That you can use such as the, so the weekdays functions tells you what day of the week a given time is or a given day is. The month function tells you what month that date or time is. And the quarters functions gives you the quarter number. So for example, quarter Q1 would be January through March, Q2 would be April through June, etc. like that. So, these generic functions operate on, on objects of class. POSIXct or POSIXlt or date. So, so, for example, you can, you can, you can coerce things back and forth between POSIXlt and POSIXct. If you want, using the as.POSIXlt or the as.POSIXct function. so, for example the Sys.time function here, just gives you the current time. As it's known by the system. And you can see that when it prints out, it prints out in a year month day format. And then an hour minute second format. And then the, the timezone, which is Eastern Standard Time right now. [COUGH] So you can convert this di to a POSIXlt using pa, as.POSIXlt. And, POSIXlt remember underlying is a list. So you can look at the names of the elements. In this list if you unclass it and you can see that there's an element called seconds that's the seconds and the minutes the hours the M days the day of the month which in this case would be 23. The month is just the month your in so that's a January and then the year. The weekdays or the day of the week. The year, day, which is the day of the year. And then are we on daylight savings or not? [NOISE] So, if I extract the sec, seconds element of this list, you'll see it's 11.86. And so that, and so this actually gives you the seconds in in fractional seconds. So, it's 11 seconds and then .86 seconds. So, that's, that's the number of seconds in the time. The POSIXct format you can see is you can also get it from the sys.time function and you can see that if I un-class the POSIXct object. I get this very large integer, because that's just the number of seconds since January 1, 1970. Now if I try to apply the list operator, the dollar sign to this object, you see I get an error because objects of POSIXct don't have these list elements in them. You want to get those list elements out you have to convert it to POSIXlt using the as.POSIXlt function. Then I can get the seconds out. In this case it's 11.88 seconds. So, finally there's a strptime function which converts dates which are written in character string format into date or time objects. Well, in this case it converts into two time objects. So and they use what are called format strings. So here I've got a string that says January 10th, 2012 and then 10, 40 meaning hour 10 minute 40 and then I have another string that said December 9th 2012. 9, 10. So if I want to convert these strings to time objects I can use the strptime function, what I do is I pass the character vector and I pass it a format string. So in this case I got, and so you'll see how these present signs fall by letters. And then you can, you can look-up what these symbols mean in the help page for strptime. So present B means the month. In an abbreviate [UNKNOWN] name, %d is the day. Then comma and then %Y is the four-digit year. Then %H is the hour, sort of like colon followed by %M which is the minute. And so that's the format. Of the, of the times here. You can see that after I call [UNKNOWN] I print out X, I get these time objects that are formatted, that are printed out in the standard format. When I look at the class of this object you'll see it's in a POSIXlt format and so that's the so you can look at so that's the underlying kind of list format here. Now I personally can never remember what those formatting strings are the %B ,%D, %Y I can never remember what those are and so I always have to look at the help page for [UNKNOWN] to remember what those details are. Now, once you've got data in the date or time format, you can, you can do operations on them, which can be very handy for example, you can you can add and subtract dates, you can compare dates, you can see is, is one date less than another date or are these two dates equal to each other? So, but the end you need to be careful that you can't always mix different classes. So for here, I have X which is a date object, then Y which is a POSIXlt object. If I try to subtract the two, you'll, you'll get an error because they're not the same, type of object. So I can, can, but if I convert the date to a POSIXlt object so I can do as.POSIXlt on it, then if I take the difference it'll say that the, the difference of 356.3 days between the two. The nice thing about the date and time operators is that they keep track of very tricky things like leap years, leap seconds daylight savings and time zones. So just, this first example here, you can see that so 2012, it was a leap year and so there was a February 29th. And so the difference between March 1st and February 28th is actually a difference of two days, not a difference of one day like it is every other year. Similarly I could take two times one which is in my, X which is in my kind of current time and then Y which is in the time zone of GMT, so Greenwich Mean Time. And take the difference between those. So even though it looks like it should be a 5-hour difference, it's actually only a 1 hour difference because of the change in the time zones. So one of the advantages of using the date time classes is that it will automatically take care of these kinds of irregularities. So that was just a quick sum, kind of overview of the, the dates and the time classes in R. So just to summerize there are special classes in R that will, that represents dates and times that'll allow you to do numerical and statistic calculations. dates, date, class, times use either the POSIXct or POSIClt class. And then character strings can be coerced to either a date or a time class, using the strptime function or as.Date.as.POSIXlt, and POSIX, as.POSIXct. The other thing to note, that I haven't really talked about here is that a lot of the plotting functions, will recognize date time objects. So when you try to plot An object that, that's a date time class. It will recognize that object and then format the X axis in a special way so that it will have a time element to it. So you might want to try to experiment with that a little bit to see how plots change when you use a date time class.
