TIL post around timezones in JS

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Lewis Dale 2024-02-12 14:53:46 +00:00
parent 58a39c4e4c
commit b2c42a8b0c
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[
{
"name": "Robb Knight",
"website": "https://rknight.me",
"rss": "https://rknight.me/subscribe/posts/rss.xml"
},
{
"name": "Terence Eden",
"website": "https://shkspr.mobi",
"rss": "https://shkspr.mobi/blog/feed/atom/"
},
{
"name": "Sara Joy",
"website": "https://sarajoy.dev",
"rss": "https://sarajoy.dev/rss.xml"
}
]

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---
title: "TIL: Why Date.parse gives unexpected results for GMT"
tags:
- Javascript
- TIL
---
TL;DR: Timezones are weird
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.
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.
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`.
[^1]: Obviously this only breaks in the UK

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---
title: Blogroll
layout: page.njk
---
<ul>
{% for blog in blogroll %}
<li>
{{ blog.name }}
</li>
{% endfor %}
</ul>