Have gnus-summary-limit-to-age operate on calendar days

Reported and patched by Łukasz Stelmach <stlman@poczta.fm>.

* lisp/gnus/gnus-sum.el (gnus-summary-limit-to-age): Rather than
24-hour blocks. Also use `read-number', and drop the confusing option
to reverse younger/older both with the prefix argument and with a
negative number.

lisp/gnus/gnus-sum.el

@@ -8331,39 +8331,29 @@ articles."
 
 (defun gnus-summary-limit-to-age (age &optional younger-p)
   "Limit the summary buffer to articles that are older than (or equal) AGE days.
-If YOUNGER-P (the prefix) is non-nil, limit the summary buffer to
-articles that are younger than AGE days."
+Days are counted from midnight to midnight, and now to the
+previous midnight counts as day one.  If YOUNGER-P (the prefix)
+is non-nil, limit the summary buffer to articles that are younger
+than AGE days."
   (interactive
-   (let ((younger current-prefix-arg)
-	 (days-got nil)
-	 days)
-     (while (not days-got)
-       (setq days (if younger
-		      (read-string "Limit to articles younger than (in days, older when negative): ")
-		    (read-string
-		     "Limit to articles older than (in days, younger when negative): ")))
-       (when (> (length days) 0)
-	 (setq days (read days)))
-       (if (numberp days)
-	   (progn
-	     (setq days-got t)
-	     (when (< days 0)
-	       (setq younger (not younger))
-	       (setq days (* days -1))))
-	 (message "Please enter a number.")
-	 (sleep-for 1)))
+   (let* ((younger current-prefix-arg)
+	  (days (read-number
+                 (if younger "Limit to articles younger than days: "
+                   "Limit to articles older than days: "))))
      (list days younger))
    gnus-summary-mode)
   (prog1
-      (let ((data gnus-newsgroup-data)
-	    (cutoff (days-to-time age))
-	    articles d date is-younger)
+      (let* ((data gnus-newsgroup-data)
+             (now (append '(0 0 0) (cdddr (decode-time))))
+             (delta (make-decoded-time :day (* -1 (- age 1))))
+             (cutoff (encode-time (decoded-time-add now delta)))
+	     articles d date is-younger)
 	(while (setq d (pop data))
 	  (when (and (mail-header-p (gnus-data-header d))
 		     (setq date (mail-header-date (gnus-data-header d))))
 	    (setq is-younger (time-less-p
-			      (time-since (gnus-date-get-time date))
-			      cutoff))
+			      cutoff
+			      (gnus-date-get-time date)))
 	    (when (if younger-p
 		      is-younger
 		    (not is-younger))