Add a date format for download names

The new setting gives the strftime format of {pubdate}, {timestamp} and {last_modified}.
The default is Calibre's "%b, %Y".

README.md

@@ -469,7 +469,7 @@ names, so a template from Calibre gives the same name in Bindery. Bindery adds `
 | `{tags}` | The tags in sorted order, with `, ` between them. |
 | `{languages}` | The language, as Bindery stores it. |
 | `{publisher}` | The story site. |
-| `{pubdate}` | The published date, in the form `May, 2020`. |
+| `{pubdate}` | The published date. |
 | `{timestamp}` | The date when Bindery added the story. |
 | `{last_modified}` | The date when the chapters last changed. |
 | `{id}` | The CWA book ID. For a story with no library book, and without CWA, the Bindery story ID. |
@@ -489,6 +489,12 @@ names, so a template from Calibre gives the same name in Bindery. Bindery adds `
 - Bindery replaces each character that a file name cannot have with `_`. A name has 100 characters
   and 200 bytes at most.
 
+"Date format" sets the form of `{pubdate}`, `{timestamp}` and `{last_modified}`. It takes the
+[codes of Python's strftime](https://docs.python.org/3/library/datetime.html#format-codes). `%d`
+gives the day, `%b` the month name, `%m` the month number and `%Y` the year. The default is
+`%b, %Y`, as in Calibre. It gives `May, 2020`. To go back to the default, clear the field and save.
+A `/` in a date becomes `_`, as in Calibre.
+
 Bindery follows Calibre 9.2.1 with its default tweaks, for English. The order of tags with letters
 outside ASCII can differ from Calibre. The template applies only to downloads in the browser. It
 does not change the names of the files in `INGEST_DIR`. The browser can ignore the name.