Does "InRelease is expired" indicate an upstream problem or am I doing something wrong?

This is my first time trying to build a custom Kali image. I'm running Debian (9.6/stretch) and following the "Building Kali on Non-Kali Debian Based Systems" section of the docs. When I try to build the image the script seems to fail before the packages are downloaded. The error message is like this:

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E: InRelease file http://archive.kali.org/kali/dists/kali-rolling/InRelease is expired since (Mon, 07 Jan 2019 00:00:00 -0600)

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This is different than the more common "expired key" messages I found while searching for a solution because it seems to be saying the file itself is expired, not the key. Is this a problem with something outside my machine or am I doing something wrong? How can I work-around or get more information about the problem?

Update: when I run this today the expiration date does indeed increment, i.e.

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E: InRelease file http://archive.kali.org/kali/dists/kali-rolling/InRelease is expired since (Tue, 08 Jan 2019 00:00:00 -0600)

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Also, I found that I can create a .mirror file to make the script use an alternate URL. If I run

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echo "http://http.kali.org" > .mirror

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and then run the build script again the error changes to this:

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E: Failed getting release file http://http.kali.org/dists/stretch/Release

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