TIL post around timezones in JS
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src/_data/blogroll.json
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src/_data/blogroll.json
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[
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{
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"name": "Robb Knight",
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"website": "https://rknight.me",
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"rss": "https://rknight.me/subscribe/posts/rss.xml"
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},
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{
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"name": "Terence Eden",
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"website": "https://shkspr.mobi",
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"rss": "https://shkspr.mobi/blog/feed/atom/"
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},
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{
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"name": "Sara Joy",
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"website": "https://sarajoy.dev",
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"rss": "https://sarajoy.dev/rss.xml"
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}
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]
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src/blog/posts/2024/2/why-is-date.parse-weird-in-gmt.md
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src/blog/posts/2024/2/why-is-date.parse-weird-in-gmt.md
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---
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title: "TIL: Why Date.parse gives unexpected results for GMT"
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tags:
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- Javascript
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- TIL
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---
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TL;DR: Timezones are weird
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Someone posted about this in a Discord server I'm on: Javascript's Date object has a parse function, [Date.parse](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/parse) that takes a string can converts it to a timestamp. If you give it a non-standard string, in this case `Jan 1, 1970`, it should still parse it.
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The problem is though, that even though we're in GMT, and `GMT == UTC`, `Date.parse("Jan 1, 1970")` results in a timestamp of `-3600000`, or Dec 31, 1969 23:00:00. If you explicitly add the GMT timezone, it's fine.
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It turns out that if you omit the timezone, `Date.parse` will use your system timezone - now in my case that's GMT _today_[^1], but if I were to go back to 1970 I'd actually find that I'd be in BST. This is because the government [experimented with scrapping daylight savings](https://www.rmg.co.uk/stories/topics/uk-time-british-summer-time-bst-daylight-saving). So, when you parse that specific date, your timezone is actually `UTC+1`.
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[^1]: Obviously this only breaks in the UK
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src/blogroll.html
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src/blogroll.html
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---
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title: Blogroll
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layout: page.njk
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---
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<ul>
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{% for blog in blogroll %}
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<li>
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{{ blog.name }}
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</li>
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{% endfor %}
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</ul>
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