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This Cache-Control header technique and the meta tag bearing Expires were introduced from HTTP/1.1 onward.

HTTP 1.1 (RFC 2068), available at http://www.w3.org/Protocols/rfc2068/rfc2068, specifies that all HTTP date/time stamps must be generated in Greenwich Mean Time (GMT) and the format should follow the rules defined in RFC 1123, that is, weekday "," date time "GMT".

weekday would bear values including Mon, Tue, Wed, Thu, Fri, Sat, or Sun.
date should be written in this format: day(2 digits) month(3 chars) year(4 digits). For example, 02 Jun 1982.
The month should be represented using Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, or Dec).
time should be formatted as Hour ":" Minutes":" Seconds, which ranges from 00:00:00 to 23:59:59.

So, an example of this would be, CONTENT="Sat, 04 Dec 2021 21:29:02 GMT".