Add section for designing tests to pass regardless of time of execution

Code-Conventions.md

@@ -67,4 +67,16 @@ Date.new(2019, 1, 2).to_formatted_s(:long)
 # One space before day of month
 Date.new(2019, 1, 20).to_formatted_s(:long)
 # => "January 20, 2019"
-```
+```
+
+### Design tests to pass regardless of time of execution
+
+A test may be run at any time of day, any day of month, and any calendar month. Make sure that the test will always pass.
+
+Some things to watch out for:
+
+* `now + 1.hour` can still be part of today, or it can be tomorrow.
+* `today - 1.day` can be in the current calendar month, or last month
+* `today + 1.day` can be part of this calendar year, or next year
+
+If needed, you may also use `Timecop` to mock the time at which the test thinks it is running.