class: left, middle, inverse background-image: url("https://live.staticflickr.com/65535/50362989122_a8ee154fea_k_d.jpg") background-size: cover # .blue[Dates 📅] ### Environmental Data Literacy --- # Date Objects When we read a date and/or time object, it is typically given in a textual form: - February 14, 2021 - Tomorrow @ noon. - Next Wednesday morning. But in `R` we need to be able to specify these these textual representations (which mean a lot to us when we read them) into objects that we can perform actual operations on. --- # --- # A Larger Data The data we will be working with consist of data from the [Rice Rivers Center](https://ricerivers.vcu.edu) which contains water and atmospheric measurements from a stream of sensors in both the James River and on the bluff overlooking the river. ```r library( readr ) url <- "https://docs.google.com/spreadsheets/d/1Mk1YGH9LqjF7drJE-td1G_JkdADOU0eMlrP01WFBT8s/pub?gid=0&single=true&output=csv" rice <- read_csv( url ) names( rice ) ``` ``` ## [1] "DateTime" "RecordID" ## [3] "PAR" "WindSpeed_mph" ## [5] "WindDir" "AirTempF" ## [7] "RelHumidity" "BP_HG" ## [9] "Rain_in" "H2O_TempC" ## [11] "SpCond_mScm" "Salinity_ppt" ## [13] "PH" "PH_mv" ## [15] "Turbidity_ntu" "Chla_ugl" ## [17] "BGAPC_CML" "BGAPC_rfu" ## [19] "ODO_sat" "ODO_mgl" ## [21] "Depth_ft" "Depth_m" ## [23] "SurfaceWaterElev_m_levelNad83m" ``` We are going to focus on the `DateTime` column specifically as these measurements are taken every 15 minutes. --- class: sectionTitle # .green[`lubridate`] ## A more refined set of time operations. --- # Raw Data -> Usable Data Data is often derived from text. - `read_csv()` must make an estimate of a column data type should be. - Estimates are often *least divisive* data type (e.g., `character`). -- Consider the `DateTime` column in the `rice` data. ```r rice$DateTime[1] ``` ``` ## [1] "1/1/2014 12:00:00 AM" ``` -- ```r class( rice$DateTime ) ``` ``` ## [1] "character" ``` --- # Date & Time Challenges We must consider the following when attempting to conduct *operations* on date and time units. 1. Many different calendars. 2. Leap days, years, seconds. 3. Time Zones (looking at you Arizona). 4. Non-consistent base units (60 seconds, 60 minutes, 24 hours, 7 days, 28/29/30/31 days, 12 months, 100 years, 10 centuries) --- # The Unix Epoch - Time Zero! .red[.center[.large[00:00:00 January 1, 1970]]] Time on computers is kept as the number of seconds since the *epoch*. It is only .blueinline[displayed] in the Gregorian, Julian, Chinese, Jewish, and other calendars. ```r Sys.time() ``` ``` ## [1] "2021-09-13 13:13:27 EDT" ``` -- ```r unclass( Sys.time() ) ``` ``` ## [1] 1631553208 ``` --- # Making Time 🕥 To convert something like 1/1/2014 12:00:00 AM from `character` to a `time` object, we need to specify the layout of the elements within the string so the functions know what to operate on. .pull-left[ - Month as 1 or 2 digits - Day as 1 or 2 digits - Year as 4 digits - a space to separate date from time - hour (not 24-hour though) - minutes in 2 digits - seconds in 2 digits - a space to separate time from timezone - timezone - / separating date objects - : separating time objects ] -- .pull-right[ ```r rice$DateTime[1] ``` ``` ## [1] "1/1/2014 12:00:00 AM" ``` Look at the help file for `?strptime` to see these and other encodings. ```r library( lubridate ) format <- "%m/%d/%Y %I:%M:%S %p" x <- parse_date_time( rice$DateTime[1], orders=format, tz="EST") x ``` ``` ## [1] "2014-01-01 EST" ``` ] --- # Making Lots of Time ```r rice$DateTime <- parse_date_time( rice$DateTime, orders=format, tz="EST") summary( rice$DateTime ) ``` ``` ## Min. 1st Qu. Median ## "2014-01-01 00:00:00" "2014-01-22 08:22:30" "2014-02-12 16:45:00" ## Mean 3rd Qu. Max. ## "2014-02-12 16:45:00" "2014-03-06 01:07:30" "2014-03-27 09:30:00" ``` -- ```r rice$DateTime[8199] - rice$DateTime[1] ``` ``` ## Time difference of 85.39583 days ``` -- ```r txt <- paste( "Entry 5000 '", rice$DateTime[5000], "' is julian ordinal day ",yday( rice$DateTime[5000] ), sep="") txt ``` ``` ## [1] "Entry 5000 '2014-02-22 01:45:00' is julian ordinal day 53" ``` --- class: middle background-image: url("images/contour.png") background-position: right background-size: auto .center[ # 🙋🏻‍♀️ Questions?  ] <p>&nbsp;</p> .bottom[ If you have any questions for about the content presented herein, please feel free to [submit them to me](mailto://rjdyer@vcu.edu) and I'll get back to you as soon as possible.]