diff --git a/elisp/chronometrist.el b/elisp/chronometrist.el
index 0d3c97c..b2356e0 100644
--- a/elisp/chronometrist.el
+++ b/elisp/chronometrist.el
@@ -81,10 +81,14 @@ file.")
   (chronometrist-sexp-current-task))
 
 (defcustom chronometrist-duration-formats
-  `((chronometrist "%h:%.2m:%z%.2s" ,(concat (make-string 7 ?\s) "-"))
-    (chronometrist-total       "%h:%.2m:%z%.2s" ,(concat (make-string 6 ?\s) "-"))
-    (report        "%h:%.2m:%z%.2s" ,(format "% 5s  " "-"))
-    (report-total  "%h:%.2m:%z%.2s" ,(format "% 5s  " "-"))
+  `((chronometrist chronometrist-format-duration-default
+                   ,(concat (make-string 7 ?\s) "-"))
+    (chronometrist-total       chronometrist-format-duration-default
+                   ,(concat (make-string 6 ?\s) "-"))
+    (report        chronometrist-format-duration-default
+                   ,(concat (make-string 5 ?\s) "-"))
+    (report-total  chronometrist-format-duration-default
+                   ,(format "% 5s  " "-"))
     (details       "%h:%.2m:%z%.2s"))
   "List specifying duration formats.
 Each element must be in the form
@@ -93,9 +97,12 @@ Each element must be in the form
 
 FIELD should be a symbol unique to this list.
 
-FORMAT-STRING should be a string acceptable to `format-seconds'.
+FORMAT-STRING should be a string acceptable to `format-seconds',
+or a procedure of two arguments (SECONDS and BLANK-STRING)
+returning such a string.
 
-If the given duration is zero, BLANK-STRING is used instead.")
+BLANK-STRING should be a string to be used if the given duration
+is zero.")
 
 (defun chronometrist-format-duration (seconds &optional field)
   "Format SECONDS as a duration string.
@@ -107,9 +114,26 @@ the same."
   (-let [(format-string blank-string)
          (alist-get (or field 'chronometrist)
                     chronometrist-duration-formats)]
-    (if (and (zerop seconds) blank-string)
-        blank-string
-      (format-seconds format-string seconds))))
+    (cond ((and (zerop seconds) blank-string)
+           blank-string)
+          ((stringp format-string)
+           (format-seconds format-string seconds))
+          ((functionp format-string)
+           (funcall format-string seconds)))))
+
+(defun chronometrist-format-duration-default (seconds)
+  (-let* (((h m s) (chronometrist-seconds-to-hms seconds))
+          (blank   (make-string 3 ?\s)))
+    (let ((h (if (zerop h) blank (format "%2d:" h)))
+          (m (cond ((and (zerop h) (zerop m))
+                    blank)
+                   ((zerop h)
+                    (format "%2d:" m))
+                   (t (format "%02d:" m))))
+          (s (if (and (zerop h) (zerop m))
+                 (format "%2d" s)
+               (format "%02d" s))))
+      (concat h m s))))
 
 (defun chronometrist-common-file-empty-p (file)
   "Return t if FILE is empty."
diff --git a/elisp/chronometrist.org b/elisp/chronometrist.org
index 2fdedc6..9ebbb66 100644
--- a/elisp/chronometrist.org
+++ b/elisp/chronometrist.org
@@ -428,10 +428,14 @@ file.")
 *** duration-formats                                             :function:
 #+BEGIN_SRC emacs-lisp
 (defcustom chronometrist-duration-formats
-  `((chronometrist "%h:%.2m:%z%.2s" ,(concat (make-string 7 ?\s) "-"))
-    (chronometrist-total       "%h:%.2m:%z%.2s" ,(concat (make-string 6 ?\s) "-"))
-    (report        "%h:%.2m:%z%.2s" ,(format "% 5s  " "-"))
-    (report-total  "%h:%.2m:%z%.2s" ,(format "% 5s  " "-"))
+  `((chronometrist chronometrist-format-duration-default
+                   ,(concat (make-string 7 ?\s) "-"))
+    (chronometrist-total       chronometrist-format-duration-default
+                   ,(concat (make-string 6 ?\s) "-"))
+    (report        chronometrist-format-duration-default
+                   ,(concat (make-string 5 ?\s) "-"))
+    (report-total  chronometrist-format-duration-default
+                   ,(format "% 5s  " "-"))
     (details       "%h:%.2m:%z%.2s"))
   "List specifying duration formats.
 Each element must be in the form
@@ -440,9 +444,12 @@ Each element must be in the form
 
 FIELD should be a symbol unique to this list.
 
-FORMAT-STRING should be a string acceptable to `format-seconds'.
+FORMAT-STRING should be a string acceptable to `format-seconds',
+or a procedure of two arguments (SECONDS and BLANK-STRING)
+returning such a string.
 
-If the given duration is zero, BLANK-STRING is used instead.")
+BLANK-STRING should be a string to be used if the given duration
+is zero.")
 
 #+END_SRC
 *** format-duration                                              :function:
@@ -457,9 +464,28 @@ the same."
   (-let [(format-string blank-string)
          (alist-get (or field 'chronometrist)
                     chronometrist-duration-formats)]
-    (if (and (zerop seconds) blank-string)
-        blank-string
-      (format-seconds format-string seconds))))
+    (cond ((and (zerop seconds) blank-string)
+           blank-string)
+          ((stringp format-string)
+           (format-seconds format-string seconds))
+          ((functionp format-string)
+           (funcall format-string seconds)))))
+#+END_SRC
+*** format-duration-default                                      :function:
+#+BEGIN_SRC emacs-lisp
+(defun chronometrist-format-duration-default (seconds)
+  (-let* (((h m s) (chronometrist-seconds-to-hms seconds))
+          (blank   (make-string 3 ?\s)))
+    (let ((h (if (zerop h) blank (format "%2d:" h)))
+          (m (cond ((and (zerop h) (zerop m))
+                    blank)
+                   ((zerop h)
+                    (format "%2d:" m))
+                   (t (format "%02d:" m))))
+          (s (if (and (zerop h) (zerop m))
+                 (format "%2d" s)
+               (format "%02d" s))))
+      (concat h m s))))
 #+END_SRC
 *** file-empty-p                                                   :reader:
 #+BEGIN_SRC emacs-lisp