How to convert Julian day and year to a date format and calculate elapsed time in R

Question

I have a matrix with separate columns for year (YYYY), Julian day, and time (HHMM). I would like to attach a column vector with the full Gregorian date ("MM/DD/YYYY") and also a column vector which will give me the elapsed time in days. How would I go about this?
To be clear, I am referring to these Julian days: http://landweb.nascom.nasa.gov/browse/calendar.html Thanks for your help.

C7 <- read.table(text="Year   Day   Time    
2015   193    915
2015   193    930
2015   195   1400
2015   195   1415", header=T) 

I would like to add these two columns:

Year   Day   Time      Date       Elapsed Time (Days)     
2015   193    915    07/12/2015      0.00     
2015   193    930    07/12/2015      0.01    
2015   195   1400    07/14/2015      2.20    
2015   195   1415    07/14/2015      2.21     

Answer 1

First, I'd convert the values into proper POSIXt date/time values. Here I paste everything in to a string, separating out the hours and minutes using some math

pdates <- with(C7, strptime(paste(Year,Day,Time %/% 100, Time %% 100), "%Y %j %H %M"))

Now we can get your desired columns by downcasting to Date and using difftime() .

format(pdates, "%m/%d/%Y")
# [1] "07/12/2015" "07/12/2015" "07/14/2015" "07/14/2015"

round(c(difftime(pdates, min(pdates), units="days")),2)
# [1] 0.00 0.01 2.20 2.21