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In most cases, we convert numeric time to POSIXct format using R. However, if we want to compare two time points, then we would prefer the numeric time format. For example, I have a date format like "2001-03-13 10:31:00",

  begin <- "2001-03-13 10:31:00"

Using R, I want to covert this into a numeric (e.g., the Julian time), perhaps something like the passing seconds between 1970-01-01 00:00:00 and 2001-03-13 10:31:00.

Do you have any suggestions?

The Julian calendar began in 45 BC (709 AUC) as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year (known at least since Hipparchus). see http://en.wikipedia.org/wiki/Julian_calendar

rfbrown
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Frank Wang
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  • I didn't downvote but I suspect it was done because of careless mismatch between your input and output. It seems possible you hoped to just remove the punctuation in a character type vector and you wanted: 20010313103100 possibly as a numeric value but at the moemnt it's very unclear. – IRTFM Jan 16 '12 at 17:35
  • Your reply is helpful. I will revise the question. – Frank Wang Jan 16 '12 at 17:38
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    Generally speaking, `as.numeric` is a good first place to turn if you want to convert something to, well, a number. Have you tried that? – joran Jan 16 '12 at 17:41
  • good reminding. thank u. – Frank Wang Jan 16 '12 at 17:50

4 Answers4

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If you just want to remove ":" , " ", and "-" from a character vector then this will suffice:

end <- gsub("[: -]", "" , begin, perl=TRUE)
#> end
#[1] "20010313103100"

You should read the section about 1/4 of the way down in ?regex about character classes. Since the "-" is special in that context as a range operator, it needs to be placed first or last.

After your edit then the answer is clearly what @joran wrote, except that you would need first to convert to a DateTime class:

 as.numeric(as.POSIXct(begin))
#[1] 984497460

The other point to make is that comparison operators do work for Date and DateTime classed variables, so the conversion may not be necessary at all. This compares 'begin' to a time one second later and correctly reports that begin is earlier:

as.POSIXct(begin) < as.POSIXct(begin) +1
 #[1] TRUE
IRTFM
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    or more generally `gsub("[^[:digit:]]", "", begin)` will remove anything other than number. – kohske Jan 16 '12 at 17:48
7

Based on the revised question this should do what you want:

begin <- "2001-03-13 10:31:00"
as.numeric(as.POSIXct(begin))

The result is a unix timestamp, the number of seconds since epoch, assuming the timestamp is in the local time zone.

BravoKing
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3

Maybe this could also work:

library(lubridate)
...
df <- '24:00:00'

as.numeric(hms(df))

hms() will convert your data from one time format into another, this will let you convert it into seconds. See full documentation.

I tried this because i had trouble with data which was in that format but over 24 hours.

Alberto_Bella
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The example from ?as.POSIX help gives

as.POSIXct(strptime(begin, "%Y-%m-%d %H:%M:%S"))

so for you it would be

as.numeric(as.POSIXct(strptime(begin, "%Y-%m-%d %H:%M:%S")))
Boern
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