Use the countdown of the http response header Date (also called format problem)

it is not the server that sends the time directly to the client, but the Date, when requesting an arbitrary file from the server to do the countdown. The problem is to get [timestamp] or [YY-MM-DD].
1. When the Date is obtained in the figure, it is Thu. The first solution for 13 Sep 2018 Sep 0849 GMT, is to convert js to the corresponding format [how to convert it?].
2. The second is whether I get the format I need directly from Date (I don"t understand the HTTP protocol either. I don"t know if I can directly set it up to get what I need).
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Jun.24,2021
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