Add Comparing DateTimes Down To Second Precision as a rails til

Author: jbranchaud

README.md

@@ -9,7 +9,7 @@ and pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://tinyletter.com/jbranchaud).
 
-_895 TILs and counting..._
+_896 TILs and counting..._
 
 ---
 
@@ -527,6 +527,7 @@ _895 TILs and counting..._
 - [Change The Nullability Of A Column](rails/change-the-nullability-of-a-column.md)
 - [Check Specific Attributes On ActiveRecord Array](rails/check-specific-attributes-on-activerecord-array.md)
 - [Code Statistics For An Application](rails/code-statistics-for-an-application.md)
+- [Comparing DateTimes Down To Second Precision](rails/comparing-datetimes-down-to-second-precision.md)
 - [Conditional Class Selectors in Haml](rails/conditional-class-selectors-in-haml.md)
 - [Convert A Symbol To A Constant](rails/convert-a-symbol-to-a-constant.md)
 - [Creating Records of Has_One Associations](rails/creating-records-of-has-one-associations.md)

rails/comparing-datetimes-down-to-second-precision.md

@@ -0,0 +1,31 @@
+# Comparing DateTimes Down To Second Precision
+
+You may have an RSpec test for your Rails codebase that asserts about the
+datetime a record gets saved with:
+
+```ruby
+two_weeks_ago = 2.weeks.ago
+
+record = Thing.create(two_weeks_ago)
+
+expect(record.some_date_time).to eq(two_weeks_ago)
+```
+
+This comparison happens with precision down to the nanosecond. Unfortunately,
+depending on your operating system and backing database, you may see
+inconsistent results due to variations in precision.
+
+One way to deal with this, if you only care about precision down to the second,
+is to modify the expecationa little with the [`be_within`
+matcher](https://relishapp.com/rspec/rspec-expectations/docs/built-in-matchers/be-within-matcher).
+
+```ruby
+two_weeks_ago = 2.weeks.ago
+
+record = Thing.create(two_weeks_ago)
+
+expect(record.some_date_time).to be_within(1.second).of(two_weeks_ago)
+```
+
+The `be_within` matcher can also be used as [a nested
+matcher](https://twitter.com/jbrancha/status/1213162124777869319?s=20).