Accuracy of nanosecond component in Ruby Time
ruby
Solution
I strongly suspect this is due to `Time.at` taking a binary floating point number - that it has nothing to do with time at all.
I'm not a Ruby dev, but in IEEE-754 64-bit floating point types, the closest value to 421.854 is exactly
421.85399999999998499333742074668407440185546875
... so my guess is that that exact value is being truncated, effectively.
(The nearest 32-bit floating point value is 421.85400390625.)
If you use a `Rational` instead to specify it as effectively an integer number of milliseconds (`Rational(421854, 1000)`), you may well find that works. Alternatively, use `at(seconds, microseconds_with_frac)`:
Time.at(421, 854000).utc.strftime('%M:%S.%L')
Problem
I have a duration in seconds with millisecond precision, 420.854 (420 seconds and 854 milliseconds). I want to format it like (Minutes):(Seconds).(Milliseconds) so I tried this: ``` Time.at(421.854).utc.strftime('%M:%S.%L') ``` The result: `"07:01.853"` As you can see it's off by one millisecond. So I dug into it further: ``` Time.at(421.854).nsec # Output: 853999999 (expected 854000000) ``` I believe the unexpected output of the above is the cause of the problem. Some numbers work as expected though for example: ``` Time.at(421.855).nsec # Output 855000000 as expected ``` Does anyone have any suggestions on solving this, the best I can come up with is to "fix" the nanosecond component afterwards with `(nanoseconds.to_f / 1000000).round * 1000000` but I feel like there might be a better approach. Really appreciate any thoughts on the subject. Thanks.