Convert number to date in Ruby

Question

I get a number representing date from a back-end, and I'm trying to convert it to a date this way:

number = item["created"].to_i
number #=> 1512387277084
DateTime.strptime(number.to_s ,'%s') #=> 49895-08-24T08:58:04+00:00
Time.at(number).utc #=> 49895-08-24 08:58:04 UTC
Time.at(number) #=> 49895-08-24 11:58:04 +0300

But this is a wrong result.

If I use Epoch & Unix Timestamp Conversion Tools, I get this result:

1512387277084
Assuming that this timestamp is in milliseconds:
GMT: Monday, December 4, 2017 11:34:37.084 AM
Your time zone: Monday, December 4, 2017 2:34:37.084 PM GMT+03:00

Time.at(number) doesn't work.

How to convert it correctly?

Answer 1

DateTime.strptime('1512387277084', '%Q')
#⇒ #<DateTime: 2017-12-04T11:34:37+00:00
#       ((2458092j,41677s,84000000n),+0s,2299161j)>

DateTime formatting.

Answer 2

According to the ruby documentation takes the at method, of the Time class, as argument "the number of seconds" since the Epoch not the "milliseconds" since the Epoch. Another variant takes as arguments the "seconds" since the Epoch and "Microseconds with frac".

You need to convert number (the input) from ms to sec and us to get the expected / correct result.

Examples

Wrong (ms interpeted as seconds):

> number = 1512387277084 # ms since epoch
=> 1512387277084
> Time.at(number)
=> 49895-08-24 10:58:04 +0200

Seconds with fractal:

> number2 = number / 1000.0
=> 1512387277.084
> Time.at(number2)
=> 2017-12-04 12:34:37 +0100
> Time.at(number2).usec
=> 84000
> Time.at(number2).nsec
=> 84000110 # Wrong of by 110 ns 

> Time.at(number2.round(3)).nsec
=> 84000110 # Wrong of by 110 ns
> Time.at(number2.round(2)).usec
=> 79999 # Wrong
> Time.at(number2.round(2)).nsec
=> 79999923 # Wrong

Seconds only:

> number3 = number / 1000
=> 1512387277
> Time.at(number3)
=> 2017-12-04 12:34:37 +0100
> Time.at(number3).usec
=> 0 # Wrong
> Time.at(number3).nsec
=> 0 # Wrong

Seconds and Microseconds:

ms = number % 1000 # Milliseconds
=> 84
> us = ms * 1000 # Microseconds
=> 84000
> Time.at(number3, us)
=> 2017-12-04 12:34:37 +0100
> Time.at(number3, us).usec
=> 84000 # OK
> Time.at(number3, us).nsec
=> 84000000 # OK
> Time.at(number3, us).to_f
=> 1512387277.084 # OK

Update 2

Seconds as Rational

> number4 = number.quo(1000)
=> (378096819271/250)
> Time.at(number4)
=> 2017-12-04 12:34:37 +0100
> Time.at(number4).usec
=> 84000 # OK
> Time.at(number4).nsec
=> 84000000 # OK
> Time.at(number4).to_f
=> 1512387277.084 # OK