Resurrect chronometrist-interval

It was used in my configuration, and has the potential to be used in
the program, too.

doc/manual.org

@@ -250,6 +250,8 @@
     5. Function - chronometrist-midnight-spanning-p (start-time stop-time)
     6. Function - chronometrist-seconds-to-hms (seconds)
        * seconds -> list-duration
+    7. Function - chronometrist-interval (event)
+       * event -> duration
 *** chronometrist-timer.el
     1. Internal Variable - chronometrist--timer-object
     2. Function - chronometrist-timer ()

elisp/chronometrist-time.el

@@ -98,6 +98,14 @@ SECONDS must be a positive integer."
          (h       (/ seconds 3600)))
     (list h m s)))
 
+(defun chronometrist-interval (event)
+  "Return the period of time covered by EVENT as a time value.
+EVENT should be a plist (see `chronometrist-file')."
+  (let ((start (plist-get event :start))
+        (stop  (plist-get event :stop)))
+    (time-subtract (parse-iso8601-time-string stop)
+                   (parse-iso8601-time-string start))))
+
 (provide 'chronometrist-time)
 
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