Add Comparing DateTimes Down To Second Precision as a rails til

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

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+# 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).