CODEWITHSUNDEEP
rdatetime
Mohit Verma
Mohit Verma

Reputation: 2089

Converting epoch to date and back to epoch

There have been questions about this earlier also here, here, here, here

Using all these info, below should have worked, but there seems some to be some translation loss.

a <- c(1433097000, 1433183400, 1433269800, 1433356200, 1433442600, 1433529000)
a
# [1] 1433097000 1433183400 1433269800 1433356200 1433442600 1433529000
b <- as.Date(as.POSIXct(a, origin='1970-01-01', tz='Asia/Kolkata'))
b <- as.numeric(as.POSIXct(b, tz='Asia/Kolkata', format="%Y-%m-%d", origin='1970-01-01'))
b
# [1] 1433030400 1433116800 1433203200 1433289600 1433376000 1433462400

==== Edit:

With tz='Asia/Kolkata', time in a is coming as 00:00:00

strftime(as.POSIXct(a, origin='1970-01-01', tz='Asia/Kolkata'), format="%Y-%m-%d %H:%M:%S")
[1] "2015-06-01 00:00:00" "2015-06-02 00:00:00" "2015-06-03 00:00:00" "2015-06-04 00:00:00" "2015-06-05 00:00:00"
[6] "2015-06-06 00:00:00"

Upvotes: 1

Views: 1473

Answers (1)

Tensibai
Tensibai

Reputation: 15784

I can't reproduce exactly as 'IST' is an unknown timezone on my machine. (R 3.2.0)

This answer is just developing @Pascal comment as you seem not getting it.

But with your aand using GMT we get dates and time at 18:30:

> as.POSIXct(a, tz='GMT', origin='1970-01-01')
[1] "2015-05-31 18:30:00 GMT" "2015-06-01 18:30:00 GMT" "2015-06-02 18:30:00 GMT" "2015-06-03 18:30:00 GMT"
[5] "2015-06-04 18:30:00 GMT" "2015-06-05 18:30:00 GMT"

Next you take the date only and then reconvert to datetime (Sorry didn't get rid of my timzeone CEST there):

> as.POSIXct(b, tz='GMT', format="%Y-%m-%d", origin='1970-01-01')
[1] "2015-05-31 02:00:00 CEST" "2015-06-01 02:00:00 CEST" "2015-06-02 02:00:00 CEST"
[4] "2015-06-03 02:00:00 CEST" "2015-06-04 02:00:00 CEST" "2015-06-05 02:00:00 CEST"

When giving a date only to as.POSIXct() the function assume it's at 00:00:00 of this day.

That's why there's is a loss, you stripped the time information, so you get a difference between your start and end objects.

Update with the specific case of 'Asia/Kolkata' of Question:

> b
[1] "2015-05-31" "2015-06-01" "2015-06-02" "2015-06-03" "2015-06-04" "2015-06-05"

as.POSIXct ignore the tz parameter when given a Date object (didn't dig on the why) so the workaround is to wrap a call to as.POSIXlt which will make a correct object with original timezone UTC at midnight:

as.POSIXlt(b)
[1] "2015-05-31 UTC" "2015-06-01 UTC" "2015-06-02 UTC" "2015-06-03 UTC" "2015-06-04 UTC" "2015-06-05 UTC"

Wrapped in as.POSIXct with the timezone it gives:

> as.POSIXct(as.POSIXlt(b),tz='Asia/Kolkata')
[1] "2015-05-31 IST" "2015-06-01 IST" "2015-06-02 IST" "2015-06-03 IST" "2015-06-04 IST" "2015-06-05 IST"

But it just changed the time-zone of the object, it did not change the hour.

I've the feeling we're on a XY problem, where this conversion is part of something else (playing with dates at some point) and that is should be handled another way with POSIXct functions like difftime or other...

Upvotes: 2

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