If-Modified-Since header support breaking Urchin?

[sulkaharo]

nightscout/cgm-remote-monitor#2393 adds support for If-Modified-Since header for Nightscout. I don't know if this is related, but upon deploying the branch the first time on a site that gets loaded by Urchin, the watchface absolutely refused to update for the first hour or so, despite Pebble watch and app restarts. Is the app using anything that might be setting the header, resulting in unexpected behavior?

[mddub]

The app sets only Cache-Control: no-cache, here, but it's possible the Pebble app is adding its own headers. The JavaScript for a Pebble app runs in a webview-like sandbox with subtle differences between iOS and Android. I'd check by logging the headers in the requests made by the watch.

Is the behavior upon deploying reproducible?