CODEWITHSUNDEEP
rdatetimetime-series
Craig
Craig

Reputation: 513

Parsing a Millisecond Timestamp To a Time In R

I'm this has been asked before, but I just can't find the exact answer. If I have a number which represents milliseconds since midnight, say 34200577, how do I turn this into an R time?

Upvotes: 4

Views: 4671

Answers (3)

Andy Pryke
Andy Pryke

Reputation: 477

A little "gottcha" which I think is worth pointing out...

In R 3.1.2 on windows 64 bit I get the following results for Dirk's example

> ISOdatetime(2013,8,22,0,0,0) + 34200577/1e3
[1] "2013-08-22 09:30:00 BST"

Note the lack of fractional seconds. This is due to the option setting for "digits.secs"

> getOption("digits.secs")
NULL

Setting this option as follows gives the expected result:

> options(digits.secs=3)
> ISOdatetime(2013,8,22,0,0,0) + 34200577/1e3
[1] "2013-08-22 09:30:00.576 BST"

As you can probably guess, this is to do with the formatting of output, not the actual values we get from our date arithmetic. See ?strptime and ?options for the documentation on this.

Upvotes: 2

IRTFM
IRTFM

Reputation: 263301

POSIXct uses 1970 as the origin of its time scale(measured in seconds.)

> time= as.POSIXct(34200577/1000 , origin=Sys.Date() )
> time
[1] "2013-08-22 02:30:00 PDT"

Note the discrepancy in results between Dirk's and my method. The POSIX times are input as assumed to occur in UCT, so there appeared the addition 8 hours for my location in UCT-8.

> difftime( as.POSIXct(34200577/1000 , origin=Sys.Date() ) , Sys.Date() )
Time difference of 9.50016 hours

You could get the time since midnight with:

 format( as.POSIXct(34200577/1000 , origin=Sys.Date(), tz="UCT" ),
          format="%H:%M:%S")
 [1] "09:30:00"

Upvotes: 2

Dirk is no longer here
Dirk is no longer here

Reputation: 368191

Construct a 'baseline time' at midnight, add the given millisecond once converted to seconds and interpret as a time:

R> as.POSIXct(as.numeric(ISOdatetime(2013,8,22,0,0,0)) + 34200577/1e3, 
+             origin="1970-01-01")
[1] "2013-08-22 09:30:00.576 CDT"
R>

In fact, the shorter

R> ISOdatetime(2013,8,22,0,0,0) + 34200577/1e3
[1] "2013-08-22 09:30:00.576 CDT"
R>

works as well as ISOdatetime() returns a proper time object which operates in fractional seconds so we just apply the given offset.

This appears to be correct as

R> 34200577/1e3       # seconds
[1] 34200.6
R> 34200577/1e3/60    # minutes
[1] 570.01
R> 34200577/1e3/60/60 # hours
[1] 9.50016
R>

Upvotes: 6

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