user53020 - 7 months ago 42

R Question

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`

`date`

202,195,189,189,197

I found a function

`as.POSIXlt`

`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 day of the year so 1st Jan is 1, 2nd Jan is 2 and so on....

Answer

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.

Source (Stackoverflow)