Dates, times and timezones can be frustrating, especially when working with environmental time series such as those collected by air and water quality sensors.
Environmental time series data often have a strong diurnal signal and are typically plotted with a time axis displaying local time. However, when data are aggregated into larger collections, it is typical to store data with a universal time axis – UTC.
Problems can arise when parsing and formatting dates and times because R defaults to the system timezone available with
Sys.timezone(). Imagine an agency scientist based in Washington, DC, using their laptop to display recent airquality data from Los Angeles while at a conference in Tasmania. The data center processing the data might be in Boudler but the data processing machine might be set to use UTC. Potential timezones (availabe with
OlsonNames()) relavent to this scenario include:
Which timezone shold be used to convert a request for data from “2019-08-08”" to “2018-08-15”" into
To enforce specification of timezones and to help with the common user interface need to specify a range of dates or times, the MazamaCoreUtils package provides the following functions:
dateRange()– parses and returns
POSIXctstart and end dates representing full days in the specified timezone
timeRange()– parses and returns
POSixctstart and end times in the specified timezone
parseDatetime()– parses and returns a vector of
POSIXctvalues in the specified timezone
Enforcing the specification of timezones throughout a body of code is the most robust way to remove timezone-related errors from your code. To help with this type of code review, the package also includes functions for testing whether specific named arguments are used with certain function calls:
To use these functions you must define a set of
function:argument rules to be applied such as:
timezoneLintRules <- list( "parse_date_time" = "tz", "with_tz" = "tzone", "now" = "tzone", "strftime" = "tz" )
This is interpreted as:
parse_date_time()function must use the
with_tz()function must use the
While these functions could be used to test for explicit use in any
function:argument pair, our concern here is primarily with specification of timezones. As an example, here is the result of linting the
dateRange.R function in this package:
> lintFunctionArgs_file("R/dateRange.R", timezoneLintRules) # A tibble: 7 x 6 file line_number column_number function_name named_args includes_required <chr> <int> <int> <chr> <list> <lgl> 1 dateRange.R 125 29 with_tz <chr > TRUE 2 dateRange.R 128 27 with_tz <chr > TRUE 3 dateRange.R 141 18 parse_date_time <chr > TRUE 4 dateRange.R 142 18 parse_date_time <chr > TRUE 5 dateRange.R 159 18 parse_date_time <chr > TRUE 6 dateRange.R 176 18 parse_date_time <chr > TRUE 7 dateRange.R 188 18 now <chr > TRUE
The result shows that the dateRange.R source code is consistent in always explicitly specifying a timezone.
Hopefully, this attention to timezones will help our code avoid misunderstandings when it comes to date and time requests.