diff --git a/resources/default_tweaks.py b/resources/default_tweaks.py
index 8d0cf3e23c..948c98e82a 100644
--- a/resources/default_tweaks.py
+++ b/resources/default_tweaks.py
@@ -157,8 +157,10 @@ sort_columns_at_startup = None
 #  mm    the minutes with a leading 0 (00 to 59) '
 #  s     the seconds without a leading 0 (0 to 59) '
 #  ss    the seconds with a leading 0 (00 to 59) '
-#  ap    use a 12-hour clock instead of a 24-hour clock, with "ap" replaced by the localized string for am or pm
-#  AP    use a 12-hour clock instead of a 24-hour clock, with "AP" replaced by the localized string for AM or PM
+#  ap    use a 12-hour clock instead of a 24-hour clock, with "ap" replaced by the lowercase localized string for am or pm
+#  AP    use a 12-hour clock instead of a 24-hour clock, with "AP" replaced by the uppercase localized string for AM or PM
+#  aP    use a 12-hour clock instead of a 24-hour clock, with "ap" replaced by the localized string for am or pm
+#  Ap    use a 12-hour clock instead of a 24-hour clock, with "AP" replaced by the localized string for AM or PM
 #  iso   the date with time and timezone. Must be the only format present
 #  For example, given the date of 9 Jan 2010, the following formats show
 #  MMM yyyy ==> Jan 2010    yyyy ==> 2010       dd MMM yyyy ==> 09 Jan 2010
diff --git a/src/calibre/utils/formatter_functions.py b/src/calibre/utils/formatter_functions.py
index 5ea730f105..6947c900de 100644
--- a/src/calibre/utils/formatter_functions.py
+++ b/src/calibre/utils/formatter_functions.py
@@ -1498,8 +1498,10 @@ The formatting codes are:
 [*]``mm   :`` the minutes with a leading 0 (00 to 59)
 [*]``s    :`` the seconds without a leading 0 (0 to 59)
 [*]``ss   :`` the seconds with a leading 0 (00 to 59)
-[*]``ap   :`` use a 12-hour clock instead of a 24-hour clock, with 'ap' replaced by the localized string for am or pm.
-[*]``AP   :`` use a 12-hour clock instead of a 24-hour clock, with 'AP' replaced by the localized string for AM or PM.
+[*]``ap   :`` use a 12-hour clock instead of a 24-hour clock, with 'ap' replaced by the lowercase localized string for am or pm.
+[*]``AP   :`` use a 12-hour clock instead of a 24-hour clock, with 'AP' replaced by the uppercase localized string for AM or PM.
+[*]``aP   :`` use a 12-hour clock instead of a 24-hour clock, with 'AP' replaced by the localized string for AM or PM.
+[*]``Ap   :`` use a 12-hour clock instead of a 24-hour clock, with 'AP' replaced by the localized string for AM or PM.
 [*]``iso  :`` the date with time and timezone. Must be the only format present.
 [*]``to_number   :`` convert the date & time into a floating point number (a `timestamp`)
 [*]``from_number :`` convert a floating point number (a `timestamp`) into an