Ruby subtracting one microsecond when converting to string

Question

I have a string timestamp:

ts = '1550989255.000300'

How can I parse ts in a Time or Datetime object as t, and have t.strftime('%6N') print correctly?

I did:

t = Time.at(ts.to_f)
t.strftime '%6N' #=> "000299"

Ruby chops off one microsecond when converting a timestamp to a string.

Just a wild guess, but might this be caused by the inaccuracies of floats? In any case, what is your question? So far, you’ve only described the issue, but not asked an actual question. — jdno, Feb 24 '19 at 07:45
@jdno I just updated my question. It is a Ruby issue, not an ActiveRecord issue. — GeekJock, Feb 24 '19 at 07:47
Possible duplicate of [Is floating point math broken?](https://stackoverflow.com/questions/588004/is-floating-point-math-broken) — Yossi, Feb 24 '19 at 07:49
Note: `ts = 1.003; t = Time.at(ts); t.strftime '%6N' #=> "002999"`. — Cary Swoveland, Feb 24 '19 at 07:59

Yossi · Accepted Answer · 2019-02-24T08:00:11.633

4

You are observing floating point math inaccuracy. If you need higher precision, you should use BigDecimal.

require 'bigdecimal'

ts = BigDecimal('1550989255.000300')
t  = Time.at(ts)
t.strftime '%6N'

=> "000300"

edited Feb 24 '19 at 08:00

answered Feb 24 '19 at 07:55

Yossi

score 2 · Answer 2 · answered Feb 24 '19 at 21:47

2

Rationals are more exact than Floats, and they are used more and more in Ruby.

ts = '1550989255.000300'
t  = Time.at(ts.to_r)  # to_r: a rational
p t.strftime '%6N'     # => "000300"

answered Feb 24 '19 at 21:47

steenslag

GeekJock · Answer 3 · 2019-02-24T08:03:49.157

1

This works:

ts = "1550989255.000300"
arr = ts.split('.')
t = Time.at(arr[0].to_i, arr[1].to_i)
t.strftime '%6N'

edited Feb 24 '19 at 08:03

answered Feb 24 '19 at 07:56

GeekJock

1

I am removing it – I think him and I typed it at the same time, I hadn't seen his when I typed mine. – GeekJock Feb 24 '19 at 08:03
We did type it simultaniously – Yossi Feb 24 '19 at 08:06
I prefer this to using `BigDecimal` for three reasons: it employs the appropriate version of `Time::at` directly (no trickery), avoids the need to load the `BigDecimal` libary and the use of a `BigDecimal` object as the argument of `Time::at` is undocumented. – Cary Swoveland Feb 24 '19 at 19:20
To reduce repetition and avoid the creation of the variable `arr` you may wish to write `Time.at(*ts.split('.').map(&:to_i)) #=> 2019-02-24 06:20:55 +0000`. – Cary Swoveland Feb 24 '19 at 19:28

3 Answers3