Add docs for pipeline duration

doc/ci/pipelines.md

@@ -36,6 +36,37 @@ Clicking on a pipeline will show the builds that were run for that pipeline.
 Clicking on an individual build will show you its build trace, and allow you to
 cancel the build, retry it,  or erase the build trace.
 
+## How the pipeline duration is calculated
+
+Total running time for a given pipeline would exclude retries and pending
+(queue) time. We could reduce this problem down to finding the union of
+periods.
+
+So each job would be represented as a `Period`, which consists of
+`Period#first` as when the job started and `Period#last` as when the
+job was finished. A simple example here would be:
+
+* A (1, 3)
+* B (2, 4)
+* C (6, 7)
+
+Here A begins from 1, and ends to 3. B begins from 2, and ends to 4.
+C begins from 6, and ends to 7. Visually it could be viewed as:
+
+```
+0  1  2  3  4  5  6  7
+   AAAAAAA
+      BBBBBBB
+                  CCCC
+```
+
+The union of A, B, and C would be (1, 4) and (6, 7), therefore the
+total running time should be:
+
+```
+(4 - 1) + (7 - 6) => 4
+```
+
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