My R use 1970-01-01:
>as.Date(15103, origin="1970-01-01")
[1] "2011-05-09"
and this matches the calculation from
>as.numeric(as.Date(15103, origin="1970-01-01"))
by the way, the zoo package, if it is loaded, overrides the base as.Date() with its own which, by default, provides origin="1970-01-01".
(i mention this in case you find that sometimes you need to add the origin, and sometimes you don't.)
If you have both date and time information in the numeric value, then use as.POSIXct. Data.table package IDateTime format is such a case. If you use fwrite to save a file, the package automatically converts date-times to idatetime format which is unix time. To convert back to normal format following can be done.
Example: Let's say you have a unix time stamp with date and time info: 1442866615
> as.POSIXct(1442866615,origin="1970-01-01")
[1] "2015-09-21 16:16:54 EDT"
Another option is the lubridate package:
library(lubridate)
x <- 15103
as_date(x, origin = lubridate::origin)
"2011-05-09"
y <- 1442866615
as_datetime(y, origin = lubridate::origin)
"2015-09-21 20:16:55 UTC"
From the docs:
Origin is the date-time for 1970-01-01 UTC in POSIXct format. This date-time is the origin for the numbering system used by POSIXct, POSIXlt, chron, and Date classes.
So generally this has been solved, but you might get this error message because the date you use is not in the correct format.
I know this is an old post, but whenever I run this I get NA all the way down my date column. My dates are in this format 20150521 – NealC Jun 5 '15 at 16:06
If you have dates of this format just check the format of your dates with:
str(sides$date)
If the format is not a character, then convert it:
as.character(sides$date)
For as.Date, you won't need an origin any longer, because this is supplied for numeric values only. Thus you can use (assuming you have the format of NealC):
as.Date(as.character(sides$date),format="%Y%m%d")
I hope this might help some of you.
Source: Stackoverflow.com