Adding BigDecimal hours to DateTime is wrong by 1 second - ruby-on-rails

Question

This happens with the update to ActiveSupport 6
start_time = DateTime.now.beginning_of_day
start_time + BigDecimal(2).hours #=> Wed, 11 Sep 2019 01:59:59 +0000
Oddly enough this works fine with Time
start_time = Time.now.beginning_of_day
start_time + BigDecimal(2).hours #=> 2019-09-11 02:00:00 +0000
Can anybody explain why?

Answer 1

Ultimately, it boils down to floating point errors in some of the math that ActiveSupport does internally.
Notice that using Rational instead of BigDecimal works:
DateTime.now.beginning_of_day + Rational(2, 1).hours
# => Mon, 02 Dec 2019 02:00:00 -0800
Time.now.beginning_of_day + Rational(2, 1).hours
# => 2019-12-02 02:00:00 -0800
Here's the relevant code from Time/DateTime/ActiveSupport:
class DateTime
def since(seconds)
self + Rational(seconds, 86400)
end
def plus_with_duration(other) #:nodoc:
if ActiveSupport::Duration === other
other.since(self)
else
plus_without_duration(other)
end
end
end
class Time
def since(seconds)
self + seconds
rescue
to_datetime.since(seconds)
end
def plus_with_duration(other) #:nodoc:
if ActiveSupport::Duration === other
other.since(self)
else
plus_without_duration(other)
end
end
def advance(options)
unless options[:weeks].nil?
options[:weeks], partial_weeks = options[:weeks].divmod(1)
options[:days] = options.fetch(:days, 0) + 7 * partial_weeks
end
unless options[:days].nil?
options[:days], partial_days = options[:days].divmod(1)
options[:hours] = options.fetch(:hours, 0) + 24 * partial_days
end
d = to_date.gregorian.advance(options)
time_advanced_by_date = change(year: d.year, month: d.month, day: d.day)
seconds_to_advance = \
options.fetch(:seconds, 0) +
options.fetch(:minutes, 0) * 60 +
options.fetch(:hours, 0) * 3600
if seconds_to_advance.zero?
time_advanced_by_date
else
time_advanced_by_date.since(seconds_to_advance)
end
end
end
class ActiveSupport::Duration
def since(time = ::Time.current)
sum(1, time)
end
def sum(sign, time = ::Time.current)
parts.inject(time) do |t, (type, number)|
if t.acts_like?(:time) || t.acts_like?(:date)
if type == :seconds
t.since(sign * number)
elsif type == :minutes
t.since(sign * number * 60)
elsif type == :hours
t.since(sign * number * 3600)
else
t.advance(type => sign * number)
end
else
raise ::ArgumentError, "expected a time or date, got #{time.inspect}"
end
end
end
end
What's happening in your case is on the t.since(sign * number * 3600) line, number is BigDecimal(2), and DateTime.since does Rational(seconds, 86400). So the whole expression when using DateTime is Rational(1 * BigDecimal(2) * 3600, 86400).
Since a BigDecimal is used as an argument to Rational, the result isn't rational at all:
Rational(1 * BigDecimal(2) * 3600, 86400)
# => 0.83333333333333333e-1 # Since there's no obvious way to coerce a BigDecimal into a Rational, this returns a BigDecimal
Rational(1 * 2 * 3600, 86400)
# => (1/12) # A rational, as expected
This value makes it back to Time#advance. Here are the results of the calculations it makes:
options[:days], partial_days = options[:days].divmod(1)
# => [0.0, 0.83333333333333333e-1] # 0 days, 2 hours
options[:hours] = options.fetch(:hours, 0) + 24 * partial_days
# => 0.1999999999999999992e1 # juuuust under 2 hours.
And finally, 0.1999999999999999992e1 * 3600 = 7199.9999999999999712, which gets floored when it's finally converted back to a time/datetime.
This doesn't happen with Time, since Time doesn't ever need to pass the duration's value into a Rational.
I don't think this should be considered a bug, since if you're passing a BigDecimal then is how you should expect the code to treat your data: As a number with a decimal component, rather than as a ratio. That is, when you use BigDecimals you open yourself up to floating point errors.

Answer 2

It's off by one second, not milisecond. Why not use 2.hours instead of BigDecimal(2).hours?