I assumed something like this header already existed because it's such an obvious need, but...

> All HTTP date/time stamps MUST be represented in Greenwich Mean Time (GMT), without exception.

according to [rfc2616](https://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3....). Presumably that makes a lot of awkward conversions unnecessary, but a separate TZ header would be a great addition.

Yeah that HTTP date format thing is kind of orthogonal - the same document slightly later explicitly says it's talking about the format used within HTTP messages/headers and not relevant to user-facing display within the page content. Which in that case makes a lot of sense because it's effectively saying "use UTC everywhere".