Use higher level helpers instead of `encode-time'

* lisp/org-clock.el (org-clock-sum)
(org-clock-update-time-maybe):
Prefer org-time-string-to-seconds to doing it by hand.
* lisp/org-macs.el (org-2ft):
Prefer org-time-string-to-seconds to doing it by hand.
* lisp/org-table.el (org-table-eval-formula):
Prefer org-time-string-to-time to doing it by hand.

Max Nikulin:
A larger patch "Improve Org usage of timestamps" was suggested in
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=54764#10

Only cosmetic changes are selected for this patch.

lisp/org-clock.el

@@ -1914,13 +1914,10 @@ PROPNAME lets you set a custom text property instead of :org-clock-minutes."
 	  (cond
 	   ((match-end 2)
 	    ;; Two time stamps.
-	    (let* ((ts (float-time
-			(apply #'encode-time
-			       (save-match-data
-				 (org-parse-time-string (match-string 2))))))
-		   (te (float-time
-			(apply #'encode-time
-			       (org-parse-time-string (match-string 3)))))
+	    (let* ((ss (match-string 2))
+		   (se (match-string 3))
+		   (ts (org-time-string-to-seconds ss))
+		   (te (org-time-string-to-seconds se))
 		   (dt (- (if tend (min te tend) te)
 			  (if tstart (max ts tstart) ts))))
 	      (when (> dt 0) (cl-incf t1 (floor dt 60)))))
@@ -3052,10 +3049,8 @@ Otherwise, return nil."
 	  (end-of-line 1)
 	  (setq ts (match-string 1)
 		te (match-string 3))
-	  (setq s (- (float-time
-		      (apply #'encode-time (org-parse-time-string te)))
-		     (float-time
-		      (apply #'encode-time (org-parse-time-string ts))))
+	  (setq s (- (org-time-string-to-seconds te)
+		     (org-time-string-to-seconds ts))
 		neg (< s 0)
 		s (abs s)
 		h (floor (/ s 3600))

lisp/org-macs.el

@@ -1358,7 +1358,7 @@ nil, just return 0."
    ((numberp s) s)
    ((stringp s)
     (condition-case nil
-	(float-time (apply #'encode-time (org-parse-time-string s)))
+	(org-time-string-to-seconds s)
       (error 0)))
    (t 0)))
 

lisp/org-table.el

@@ -2607,8 +2607,7 @@ location of point."
 		     (format-time-string
 		      (org-time-stamp-format
 		       (string-match-p "[0-9]\\{1,2\\}:[0-9]\\{2\\}" ts))
-		      (apply #'encode-time
-			     (save-match-data (org-parse-time-string ts))))))
+		      (save-match-data (org-time-string-to-time ts)))))
 		 form t t))
 
 	  (setq ev (if (and duration (string-match "^[0-9]+:[0-9]+\\(?::[0-9]+\\)?$" form))