Incorrect Last-Modified date and out-of-date data on Firefox Ajax request

[stuartthomas25]

Describe the bug
The response sometimes includes future last-modified dates and does not provide up-to-date data in Firefox.

To Reproduce
I access https://api.weather.gov/gridpoints/AKQ/72,68/forecast in Firefox using an Javascript ajax request. Other browsers (Safari & Chrome), as well as accessing the url directly, yield correct data.

I can reproduce this bug on multiple devices and a Firefox derivative browser, Palemoon. Furthermore, this is independent of local caches since it appears after all caches are cleared.

Here are the request and response headers:

Request:

GET /gridpoints/AKQ/72,68/forecast HTTP/2
Host: api.weather.gov
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:79.0) Gecko/20100101 Firefox/79.0
Accept: */*
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate, br
Origin: null
Connection: keep-alive
Pragma: no-cache
Cache-Control: no-cache
TE: Trailers

Response:

HTTP/2 200 OK
server: nginx/1.16.1
content-type: application/geo+json
last-modified: Fri, 21 Aug 2020 13:00:00 GMT
access-control-allow-origin: *
access-control-allow-headers: Feature-Flags
x-request-id: 7df3e9f8-2e0a-4397-9649-78cb336f14ee
x-correlation-id: 2f855d41
x-server-id: vm-bldr-nids-apiapp12.ncep.noaa.gov
content-encoding: gzip
content-length: 1571
cache-control: public, max-age=891, s-maxage=3600
expires: Tue, 18 Aug 2020 06:26:26 GMT
date: Tue, 18 Aug 2020 06:11:35 GMT
vary: Accept-Encoding
x-edge-request-id: 34060756
vary: Accept,Feature-Flags,Accept-Language
strict-transport-security: max-age=31536000 ; includeSubDomains ; preload
X-Firefox-Spdy: h2

You can see above how the last-modified field is in the future.

Furthermore, the JSON response has an out-of-date list of periods, as shown by this snippet of the response:

...
       "periods": [
            {
                "number": 1,
                "name": "This Afternoon",
                "startTime": "2020-08-17T15:00:00-04:00",
                "endTime": "2020-08-17T18:00:00-04:00",
                "isDaytime": true,
                "temperature": 83,
                "temperatureUnit": "F",
                "temperatureTrend": null,
                "windSpeed": "2 mph",
                "windDirection": "S",
                "icon": "https://api.weather.gov/icons/land/day/tsra_hi,20?size=medium",
                "shortForecast": "Slight Chance Showers And Thunderstorms",
                "detailedForecast": "A slight chance of showers and thunderstorms before 4pm. Mostly sunny, with a high near 83. South wind around 2 mph. Chance of precipitation is 20%."
            },
...

This demonstrates that the API is serving data that should end at 2020-08-17T18:00:00-04:00 in a request that was made Tue, 18 Aug 2020 06:11:35 GMT.

[qconrad]

I've also noticed the last-modified and the "If-Modified-Since" header are sometimes unreliable