Converting unix seconds in milliseconds to POSIXct/POSIXlt

Question

Why do I see a difference when I convert a unix timestamp to datetime object in R?

> as.POSIXlt(1268736919, origin="1970-01-01", tz="America/New_York")
[1] "2010-03-16 06:55:19 EDT"

> as.POSIXct(1268736919, origin="1970-01-01", tz="America/New_York")
[1] "2010-03-16 11:55:19 EDT"

The result from POSIXlt is actually correct.

Also, is there a way to do this conversion without specifying the origin?

Thanks

Answer 1

The help page actually hints at a difference:

Value:

     ‘as.POSIXct’ and ‘as.POSIXlt’ return an object of the appropriate
     class.  If ‘tz’ was specified, ‘as.POSIXlt’ will give an
     appropriate ‘"tzone"’ attribute.

This stuff is finicky -- I think there is an implicit TZ conversion happening for as.POSIXct. Consider that

R> print(as.numeric(as.POSIXct(as.POSIXlt(1268736919, 
                               origin="1970-01-01"))), digits=10)
[1] 1268736919
R> print(as.numeric(as.POSIXct(1268736919, origin="1970-01-01")), digits=10)
[1] 1268758519

the second one (using as.POSIXct) does not return the original input. Unfortunately, Brian D. Ripley seems to be the only human having all the details here.

Lastly, you can't do it without the origin. But you could define wrappers that use the epoch as origin (as here) or use 2000-01-01 or ... Just keep it consistent.