Add Duration Arithmetic guideline

fixes #282

Author: pirj

README.adoc

@@ -1662,6 +1662,21 @@ Avoid using negative numbers for the duration subject. Always prefer using a qua
 5.hours.from_now
 ----
 
+=== Duration Arithmetic
+
+Use Duration methods instead of adding and subtracting with the current time.
+
+[source,ruby]
+----
+# bad
+Time.current - 1.minute
+Time.zone.now + 2.days
+
+# good
+1.minute.ago
+2.days.from_now
+----
+
 == Bundler
 
 === Dev/Test Gems [[dev-test-gems]]