Why is Date.today - 6.months + 6.months != Date.today?

Question

In Ruby, on Halloween:

Date.today - 6.months + 6.months != Date.today

Do we need to update Ruby\'s date implementation? Do other languages

Answer 1

This is a weird feature of ActiveSupport time extensions.

> 6.months == 180.days
=> true 

but when you do date math with months, they are considered calendar months, not 30-day periods. Check this out:

> Date.today - 180.days + 180.days
=> Mon, 31 Oct 2011 

See https://github.com/rails/rails/blob/master/activesupport/lib/active_support/core_ext/time/calculations.rb#L277 and dig further from there.