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The title has it: how do you convert a POSIX date to day-of-year?
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POSIXct stores date and time in seconds with the number of seconds beginning at 1 January 1970. Negative numbers are used to store dates prior to 1970. Thus, the POSIXct format stores each date and time a single value in units of seconds. Storing the data this way, optimizes use in data.
The builtin as. Date function handles dates (without times); the contributed library chron handles dates and times, but does not control for time zones; and the POSIXct and POSIXlt classes allow for dates and times with control for time zones.
To get the year from a date in R you can use the functions as. POSIXct() and format() . For example, here's how to extract the year from a date: 1) date <- as. POSIXct("02/03/2014 10:41:00", format = "%m/%d/%Y %H:%M:%S) , and 2) format(date, format="%Y") .
An alternative is to format the "POSIXt" object using strftime():
R> today <- Sys.time() R> today [1] "2012-10-19 19:12:04 BST" R> doy <- strftime(today, format = "%j") R> doy [1] "293" R> as.numeric(doy) [1] 293 which is preferable to remembering that the day of the years is zero-based in the POSIX standard.
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