rendering date/times from 64bit julian format?

Derek Eder <derek.eder at lungall.gu.se> writes:

Hellos,

I have date/times in 64-bit Julian format, that is the number of 
milliseconds since 1 jan 1970 (or something like that).    E.g.,   
"1159884877406"

For the life of me I can't figure out how to render these in a more 
readable format, e.g.,  "2006-10-04  23:12:93.191"  (and I have tried to 
do my best searching the archives and help etc ...)

This should be close:

format(ISOdatetime(1970,1,1,0,0,0)+1159884877406/1000,"%Y-%m-%d %H:%M:%OS3")

[1] "2006-10-03 15:14:37.406"

Beware the timezone issues though.

O__  ---- Peter Dalgaard             ?ster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45) 35327907

| I have date/times in 64-bit Julian format, that is the number of 
| milliseconds since 1 jan 1970 (or something like that).    E.g.,   
| "1159884877406"
| 
| For the life of me I can't figure out how to render these in a more 
| readable format, e.g.,  "2006-10-04  23:12:93.191"  (and I have tried to 
| do my best searching the archives and help etc ...)

To paraphrase an old saying about Unix, you could say that R's very powerful
Date/Time operations are indeed very user-friendly -- but unfortunately also
picky in selecting their friends.

To key to this conversion is to use implicit casting.  Witness

now <- Sys.time()
class(now)

[1] "POSIXt"  "POSIXct"

format(as.numeric(now), digits=16)

[1] "1159968337.833141"

so we *do* have the current time in a POSIXct as such a number.  So for your
purposes, create an 'offset', maybe via 

 > offset <- ISOdatetime(1970,1,1,0,0,0,tz="GMT")

class(offset)

[1] "POSIXt"  "POSIXct"

as.numeric(offset)

[1] 0

which gives you half the solution -- a POSIXct to start from, conveniently
placed at the 'epoch.. Pick whichever timezone works for you.

Then simply add your milliseconds -- but converted to seconds as that is how
the internal representation is scaled:

offset + 1159884877406/1000

[1] "2006-10-03 14:14:37.406 GMT"

class(offset + 1159884877406/1000)

[1] "POSIXt"  "POSIXct"

Now your returned object is still POSIXct so you get to do all sort of fany
conversions for free.

The really nice thing is that thanks for a number of post-R 2.3.1
enhancements by Brian Ripley, we do have reall milisecond granularity in R.

Hope this helps,  Dirk

PS  Kurt, would this be worthy of a new FAQ entry?

Hell, there are no rules here - we're trying to accomplish something. 
                                                  -- Thomas A. Edison