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Add principle on use of delta seconds #467

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Add principle on use of delta seconds #467

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Expand Up @@ -2334,6 +2334,21 @@ and there is no way to make them immutable.
on the Web IDL Bugzilla
</div>

<h3 id="use-delta-seconds">Use Delta-Seconds over HTTP-Date</h3>

For HTTP header-based APIs, using Delta-seconds is prefered over HTTP-Date for a few reasons:

* Parsing is inconsistent, especially around error handling
(see <a href="https://cache-tests.fyi/#expires-parse">tests</a>)
* parsing is relatively expensive
* To adjust for clock sync, you need to have a Date header available (which
means you need to parse two dates)
* Because of that, potential drift is doubled to two seconds

Delta-seconds are much more straightforward. Developers sometimes ask for sub second
resolution in HTTP headers, but this is typically not accommodated for in specs,
since the RTT for a request can easily make that meaningless at best and a footgun at worst.

<h3 id="error-types">Use Error or DOMException for errors</h3>

Represent errors in web APIs as ECMAScript error objects (e.g., {{Error}})
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