Default: "!%a, %b %d, %Y at %I:%M:%S%p %Z"
This variable controls the format of the date printed by the ``%d'' sequence in `` $index_format''. This is passed to the strftime call to process the date. See the man page for strftime(3) for the proper syntax.
Unless the first character in the string is a bang (``!''), the month and week day names are expanded according to the locale specified in the variable `` $locale''. If the first character in the string is a bang, the bang is discarded, and the month and week day names in the rest of the string are expanded in the C locale (that is in US English).