[Feature Request] Provide the Date/Time information from the "Last-Modified" HTTP header field as a fallback {date} keyword

[misc01]

When a URL doesn't provide a {date} keyword, make the Date/Time information from the "Last-Modified" HTTP header field available as fallback {date} keyword.

gallery-dl -K https://i.redd.it/lz8ptshduw651.jpg
Keywords for directory names:
-----------------------------
category
  reddit
extension
  jpg
filename
  lz8ptshduw651
subcategory
  image

Keywords for filenames and --filter:
------------------------------------
category
  reddit
extension
  jpg
filename
  lz8ptshduw651
subcategory
  image

curl -s -v --head https://i.redd.it/lz8ptshduw651.jpg 2>&1 | grep '^< Last-Modified:'
< Last-Modified: Wed, 24 Jun 2020 19:35:27 GMT

gallery-dl -K https://dto9r5vaiz7bu.cloudfront.net/roh2exbd55ngr/source.png 
Keywords for directory names:
-----------------------------
category
  directlink
domain
  dto9r5vaiz7bu.cloudfront.net
extension
  png
filename
  source
fragment
  None
path
  roh2exbd55ngr
query
  None
subcategory


Keywords for filenames and --filter:
------------------------------------
category
  directlink
domain
  dto9r5vaiz7bu.cloudfront.net
extension
  png
filename
  source
fragment
  None
path
  roh2exbd55ngr
query
  None
subcategory

curl -s -v --head https://dto9r5vaiz7bu.cloudfront.net/roh2exbd55ngr/source.png 2>&1 | grep '^< Last-Modified:'
< Last-Modified: Mon, 20 Jan 2020 16:27:20 GMT

[mikf]

Nice idea, but not easily doable with gallery-dl's current infrastructure, at least not in general. A filename usually gets built before starting an HTTP request to download it in order to potentially skip it altogether, so the Last Modified header would only be available when it's already too late.

There is currently an exception to this for missing filename extensions. Maybe I can extend this a bit an also use for date fields.