Repository: yt-dlp
Issue: Parallel download (-N) breaks server "Last modified" mtime
Labels: bug,core:downloader

### DO NOT REMOVE OR SKIP THE ISSUE TEMPLATE

- [X] I understand that I will be **blocked** if I remove or skip any mandatory\* field

### Checklist

- [X] I'm reporting a bug unrelated to a specific site
- [X] I've verified that I'm running yt-dlp version **2022.11.11** ([update instructions](https://github.com/yt-dlp/yt-dlp#update)) or later (specify commit)
- [X] I've checked that all provided URLs are playable in a browser with the same IP and same login details
- [X] I've checked that all URLs and arguments with special characters are [properly quoted or escaped](https://github.com/yt-dlp/yt-dlp/wiki/FAQ#video-url-contains-an-ampersand--and-im-getting-some-strange-output-1-2839-or-v-is-not-recognized-as-an-internal-or-external-command)
- [X] I've searched the [bugtracker](https://github.com/yt-dlp/yt-dlp/issues?q=) for similar issues **including closed ones**. DO NOT post duplicates
- [X] I've read the [guidelines for opening an issue](https://github.com/yt-dlp/yt-dlp/blob/master/CONTRIBUTING.md#opening-an-issue)

### Provide a description that is worded well enough to be understood

If I download a video using `yt-dlp https://example.com/example.mpd`, everything works - the resulting "Last modified" timestamp of the downloaded file is set to the server's "Last Modified" time reported by the server (usually the date a video was uploaded). 

If I use `-N 8` for faster downloads, that no longer works - the downloaded file will have the current date and time as "Last modified". 

I am running yt-dlp 2022.11.11 from pip (8b64402).

Is there a way I can download faster, with multiple threads, while still keeping the correct file time from the server? The help section for `--concurrent-fragments` doesn't mention anything like that. Even when I explicitly set `--mtime` (which is the default according to the help), the timestamp is still the time of download, and not the server time. 

Example: 

Running `yt-dlp https://dash.akamaized.net/dash264/TestCasesIOP33/adapatationSetSwitching/5/manifest.mpd` creates a file that's timestamped to 2017-11-28 (which is when that mpd file was "created" on the server) - which is what I'd like to get.

Running `yt-dlp -N 8 https://dash.akamaized.net/dash264/TestCasesIOP33/adapatationSetSwitching/5/manifest.mpd` creates a file that's timestamped to today, and ignores the server's timestamp.

### Provide verbose output that clearly demonstrates the problem

- [X] Run **your** yt-dlp command with **-vU** flag added (`yt-dlp -vU <your command line>`)
- [X] Copy the WHOLE output (starting with `[debug] Command-line config`) and insert it below

### Complete Verbose Output

```shell
[debug] Command-line config: ['-vU', '-N', '16', 'https://dash.akamaized.net/dash264/TestCasesIOP33/adapatationSetSwitching/5/manifest.mpd']
[debug] Encodings: locale UTF-8, fs utf-8, pref UTF-8, out utf-8, error utf-8, screen utf-8
[debug] yt-dlp version 2022.11.11 [8b64402] (pip)
[debug] Python 3.8.10 (CPython x86_64 64bit) - Linux-5.13.0-52-generic-x86_64-with-glibc2.29 (OpenSSL 1.1.1f  31 Mar 2020, glibc 2.31)
[debug] exe versions: ffmpeg N-109005-g649d7cbda8 (fdk,setts), ffprobe N-109005-g649d7cbda8, rtmpdump 2.4
[debug] Optional libraries: Cryptodome-3.12.0, brotli-1.0.7, certifi-2022.05.18.1, mutagen-1.45.1, pyxattr-0.6.1, secretstorage-3.3.1, sqlite3-2.6.0, websockets-10.0
[debug] Proxy map: {}
[debug] Loaded 1723 extractors
[debug] Fetching release info: https://api.github.com/repos/yt-dlp/yt-dlp/releases/latest
Latest version: 2022.11.11, Current version: 2022.11.11
yt-dlp is up to date (2022.11.11)
[debug] [generic] Extracting URL: https://dash.akamaized.net/dash264/TestCasesIOP33/adapatationSetSwitching/5/manifest.mpd
[generic] manifest: Downloading webpage
WARNING: [generic] Falling back on generic information extractor
[generic] manifest: Extracting information
[debug] Identified a DASH manifest
[debug] Formats sorted by: hasvid, ie_pref, lang, quality, res, fps, hdr:12(7), vcodec:vp9.2(10), channels, acodec, filesize, fs_approx, tbr, vbr, abr, asr, proto, vext, aext, hasaud, source, id
[debug] Default format spec: bestvideo*+bestaudio/best
[info] manifest: Downloading 1 format(s): 4+5
[debug] Invoking dashsegments downloader on "https://dash.akamaized.net/dash264/TestCasesIOP33/adapatationSetSwitching/5/manifest.mpd"
[dashsegments] Total fragments: 369
[download] Destination: manifest [manifest].f4.mp4
WARNING: The download speed shown is only of one thread. This is a known issue and patches are welcome
[download] 100% of  180.90MiB in 00:00:13 at 13.19MiB/s
[debug] Invoking dashsegments downloader on "https://dash.akamaized.net/dash264/TestCasesIOP33/adapatationSetSwitching/5/manifest.mpd"
[dashsegments] Total fragments: 372
[download] Destination: manifest [manifest].f5.m4a
WARNING: The download speed shown is only of one thread. This is a known issue and patches are welcome
[download] 100% of    2.99MiB in 00:00:07 at 428.17KiB/s
[Merger] Merging formats into "manifest [manifest].mp4"
[debug] ffmpeg command line: ffmpeg -y -loglevel repeat+info -i 'file:manifest [manifest].f4.mp4' -i 'file:manifest [manifest].f5.m4a' -c copy -map 0:v:0 -map 1:a:0 -movflags +faststart 'file:manifest [manifest].temp.mp4'
Deleting original file manifest [manifest].f4.mp4 (pass -k to keep)
Deleting original file manifest [manifest].f5.m4a (pass -k to keep)
```