Steps to reproduce:
click new profile of an older version say 1.2.2
click play
go to %appdata%/.minecraft/versions/1.2.2 notice the timestamp says 2020 on last modified instead of the timestamp on the json either from "releaseTime" or "time" It's suppose to say 2012 or something like that

This doesn't just effect the minecraft client jar. It effects all libraries, all minecraft individual version json files, all minecraft assets. The assets use to use minecraft.net or amazon aws depending on the minecraft version to get the timestamp and set it from that info.The libraries are also not set to the last modified timestamp as it doesn't exist. you download any file from the launcher it will always say the current date. There is evidence of intent by there being a "time" tag in the client.json for at least on the client.jar which doesn't get used/set. And I think when they converted to post 1.5.2 aka 1.6+ they forgot about setting the assets to the last modified date of the file and adding it to the assets index. the libraries never getting set to the last modfied was always an issue.

Minecraft launchers years ago use to download the audio files with the correct timestamp. I see you guys have tried to do this at least for the client.jar but, it's not getting applied even though it's in your json api. 

Edit:
And to be perfectly clear the timestamp no longer exists for audio and I believe in the alpha days and perhaps even pre 1.6 launcher days existed. I think it would be a good idea to preserve the assets timestamps so you can know when the file was released. URLConnection#lastModified or add them to your jsons as a long timestamp and set it that way. So I consider it a bug because it use to be intended to preserve the last modified of the original audio files. As for the client jar and possibly other assets it looks like you guys started with the timestamps but, never set them to the file after it's done downloading. And if your doing it for assets/jars why not also apply the same download method to grab them from your libraries as well. I already confirmed with my own download java code that you can do this with the libs as well with their original timestamps on your amazon aws servers

If you guys no longer wish to preserve the integrity of the download date then close this issue. If not then please fix this when you have time.