Convert date without year into Julian day (number of days since start of the year)

问题

I have some date that I need to convert into Julian days. This is the data I have

date <- c("21-Jul", "14-Jul", "08-Jul", "08-Jul","16-Jul")
class(date)

[1] "character"

I want to convert date into julian days. So for the above dates, the Julian days will be:

date
202,195,189,189,197 

I found a function as.POSIXlt that converts date into Julian day. For e.g.

tmp <- as.POSIXlt("16Jun10", format = "%d%b%y")
tmp$yday
# [1] 166

But this needs date in a particular order including year which I do not have. Is there any way to convert my dates which are characters and do not have year in it into Julian?

Also, this year is not a leap year.

Julian days imply the day of the year so 1st Jan is 1, 2nd Jan is 2 and so on....

回答1:

base R has a function julian that can produce what you are looking for together with paste0 and as.Date.

julian(as.Date(paste0("1970-", date), format="%Y-%d-%b"))
[1] 201 194 188 188 196
attr(,"origin")
[1] "1970-01-01"

Note that this is counted as number of days from January 1, 1970 (it starts at 0), so you can add 1 to get the desired result.

julian(as.Date(paste0("1970-", date), format="%Y-%d-%b")) + 1
[1] 202 195 189 189 197

I manually checked that 1970 is not a leap year:

seq(as.Date("1970-02-25"), as.Date("1970-03-02"), by="day")
[1] "1970-02-25" "1970-02-26" "1970-02-27" "1970-02-28" "1970-03-01" "1970-03-02"

For future reference, it is also possible to set the starting point yourself, using the origin argument to as.Date:

julian(as.Date(paste0("1980-", date), format="%Y-%d-%b"), origin=as.Date("1980-01-01"))
[1] 202 195 189 189 197
attr(,"origin")
[1] "1980-01-01"

As should be clear, origin can be set to any date that you wish, so that this could be used to calculate the number of days following (or preceding) any date.

来源：https://stackoverflow.com/questions/40656488/convert-date-without-year-into-julian-day-number-of-days-since-start-of-the-yea