Fix elfeed-time-duration-absolute for Emacs 24.3

The time-subtract function in Emacs 24.5 and earlier does not support
float seconds. So, instead, always use float seconds and replace the
time-subtract with a plain substraction (-). Emacs' encoding of time
using a list of small integers was a mistake since double precision
floats provide more than enough precision (53 bits) to store these time
values.

Also added an optional argument to support testing since overriding
float-time via cl-letf wasn't working.

elfeed-lib.el

@@ -36,13 +36,16 @@
     (end-of-line)
     (delete-region start (point))))
 
-(defun elfeed-time-duration (time)
+(defun elfeed-time-duration (time &optional now)
   "Turn a time expression into a number of seconds. Uses
-`timer-duration' but allows a bit more flair."
+`timer-duration' but allows a bit more flair.
+
+If `now' is non-nil, use it as the current time (`float-time'). This
+is mostly useful for testing."
   (cond
    ((numberp time) time)
    ((let ((iso-time (elfeed-parse-simple-iso-8601 time)))
-      (when iso-time (float-time (time-subtract (current-time) iso-time)))))
+      (when iso-time (- (or now (float-time)) iso-time))))
    ((string-match-p "[[:alpha:]]" time)
     (let* ((clean (replace-regexp-in-string "\\(ago\\|old\\|-\\)" " " time))
            (duration (timer-duration clean)))

tests/elfeed-lib-tests.el

@@ -26,17 +26,23 @@
 
 (ert-deftest elfeed-time-duration-absolute ()
   ;; fixed time for testing: assume U.S. eastern
-  (cl-letf (((symbol-function 'current-time)
-             (lambda () (encode-time 0 20 13 24 6 2019 (* -1 4 60 60)))))
+  (let ((now (float-time (encode-time 0 20 13 24 6 2019 (* -1 4 60 60)))))
     ;; "2019-06-24T13:20:00-04:00" is "2019-06-24T17:20:00Z" so 17h 20mins is
     ;; the time difference:
-    (should (= (+ (* 17 60 60) (* 20 60)) (elfeed-time-duration "2019-06-24")))
-    (should (= (* 10 60) (elfeed-time-duration "2019-06-24T17:10")))
-    (should (= (* 10 60) (elfeed-time-duration "2019-06-24T17:10:00")))
-    (should (= (+ (* 9 60) 30) (elfeed-time-duration "2019-06-24T17:10:30")))
-    (should (= (+ (* 9 60) 30) (elfeed-time-duration "2019-06-24T17:10:30Z")))
-    (should (= (+ (* 9 60) 30) (elfeed-time-duration "2019-06-24T17:10:30+00:00")))
-    (should (= (+ (* 9 60) 30) (elfeed-time-duration "20190624T17:10:30+00:00")))))
+    (should (= (+ (* 17 60 60) (* 20 60))
+               (elfeed-time-duration "2019-06-24" now)))
+    (should (= (* 10 60)
+               (elfeed-time-duration "2019-06-24T17:10" now)))
+    (should (= (* 10 60)
+               (elfeed-time-duration "2019-06-24T17:10:00" now)))
+    (should (= (+ (* 9 60) 30)
+               (elfeed-time-duration "2019-06-24T17:10:30" now)))
+    (should (= (+ (* 9 60) 30)
+               (elfeed-time-duration "2019-06-24T17:10:30Z" now)))
+    (should (= (+ (* 9 60) 30)
+               (elfeed-time-duration "2019-06-24T17:10:30+00:00" now)))
+    (should (= (+ (* 9 60) 30)
+               (elfeed-time-duration "20190624T17:10:30+00:00" now)))))
 
 (ert-deftest elfeed-format-column ()
   (should (string= (elfeed-format-column "foo" 10 :right) "       foo"))