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

i have matrix separate columns year (yyyy), julian day, , time (hhmm). attach column vector full gregorian date ("mm/dd/yyyy") , column vector give me elapsed time in days. how go this?
clear, referring these julian days: http://landweb.nascom.nasa.gov/browse/calendar.html help.

c7 <- read.table(text="year   day   time     2015   193    915 2015   193    930 2015   195   1400 2015   195   1415", header=t)  

i add these 2 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      

first, i'd convert values proper posixt date/time values. here paste in string, separating out hours , minutes using math

pdates <- with(c7, strptime(paste(year,day,time %/% 100, time %% 100), "%y %j %h %m")) 

now can desired columns downcasting date , 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