31:[] 2f:{"title":"Spacetime - Daylight Savings Time","cells":[{"id":0,"value":"md`# Spacetime - Daylight Savings Time\n\nDaylight Savings introduces challenges to timekeeping, because it changes the alignment between \\`epoch\\` time, and *calendar* time.\n\nThis can make even simple date operations error-prone, if each operation is not quality-checked carefully.\n\n[Spacetime](https://observablehq.com/@spencermountain/spacetime) depends on a **javascipt Date object** running in a different timezone, so it is subject to these problems, arguably even more than other solutions.\n\n\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":45,"value":"md`\n---\n#### Which day of the year?\nDST changes at different times each year, but spacetime does not store past DST changes, or project future ones. It interprets each year with *the current year's* DST dates.\n\n\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":22,"value":"md`\n---\nExample: **America/Chicago in 2020**\n\n> Enters DST (-5h) on - **Mar 8th @ 2:00am**\n\n> Leaves DST (-6h) on - **Nov 1st @ 2:00am**\n\n---\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":7,"value":"md`\n## Spring - DST\nIn Chicago, DST is dropped at **2am on March 8th**.\n\nThe time jumps from **1:59am** to **3:00am**, and the whole hour of *2:00am* is skipped-over.\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":60,"value":"{\n let justBefore = Object.assign(spring, {hour:0, minute:0})\n let d = spacetime(justBefore, 'America/Chicago') // 12:01am, March 8th\n let results = []\n for (let i = 0; i < 8; i += 1){\n d = d.add(20, 'minutes')\n let isDST = d.timezone().current.isDST\n results.push([d.format('nice'), isDST ? 'DST' : 'standard'])\n }\n return html`${results.map(o=>'

'+o[0]+' - '+o[1]+'

')}`\n}","pinned":false,"hidden":false,"mode":"ojs"},{"id":88,"value":"md`Spring dst-changes work pretty well. You should be able to dance back-forth across it without causing too-much trouble.\n\n#### Comparisons over dst:\nYou should also be able to compare two times between a DST change with mostly-expected behaviour:\n\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":161,"value":"{\n // 1:50am\n let before = spacetime('2020-03-08T01:50:00', 'America/Chicago')\n // 3:10am\n let after = spacetime('2020-03-08T03:10:00', 'America/Chicago')\n return after.since(before,'minutes').precise\n}","pinned":true,"hidden":false,"mode":"ojs"},{"id":157,"value":"md`\n#### Setting ambigous hours:\nSpring DST only gets a little complicated when you *create a date* in the **2am** hour:`","pinned":false,"hidden":false,"mode":"ojs"},{"id":93,"value":"// set 2:30am on March 8th\nspacetime('2020-03-08T02:30:00', 'America/Chicago').format('time') + ' 🤔'","pinned":true,"hidden":false,"mode":"ojs"},{"id":43,"value":"md`\nyou'll see some jumping-around of the date when you try to make it 2am on this date. \n\nThis often looks like a bug, but really, spacetime is squirming under an ambiguous situation, and tries to render a valid time near the change.\n\n---`","pinned":false,"hidden":false,"mode":"ojs"},{"id":5,"value":"md`\n\n## Fall - DST\nIn Chicago, DST begins at **2am on Nov 1st**.\n\nThe time goes from **1:59am** *backwards* to **1:00am**\n\nThe hour of 1am happens twice, and this is mighty-confusing.\n\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":139,"value":"{\n let justBefore = Object.assign(fall, {hour:0, minute:0})\n let d = spacetime(justBefore, 'America/Chicago') // 12:00am, March 8th\n let results = []\n for (let i = 0; i < 9; i += 1){\n d = d.add(20, 'minutes')\n let isDST = d.timezone().current.isDST\n results.push([d.format('nice'), isDST ? 'DST' : 'standard'])\n }\n return html`${results.map(o=>'

'+o[0]+' - '+o[1]+'

')}`\n}","pinned":false,"hidden":false,"mode":"ojs"},{"id":154,"value":"md`\nFall DST is particularly confusing. Sometimes spacetime has a tough time with it:\n\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":198,"value":"md`\n\n#### Setting ambiguous hours:\nAnother tricky part with fall-DST is that since there's an hour that happens twice, it's sometimes not clear *which one* you are mean:\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":191,"value":"{ \n let s = spacetime('2020-11-01T02:30:00', 'America/Chicago') // 2:30am on Nov 1st\n return s.format('time') + ' DST:'+s.timezone().current.isDST+' 🤔'\n}","pinned":true,"hidden":false,"mode":"ojs"},{"id":206,"value":"md`this is a bit of a helpless situation as well. Because of the way spacetime reckons date-time, you can get *either 2am*, based on the time of the local computer.\n\nWithin an hour of the DST-border, you may find some off-by-one errors in the fall dst-change, but spacetime will never mis-interpret things more than that, or let a off-by-one error cascade a larger change.\n\n#### Comparisons over dst:\nLastly, comparisons over this DST-change should work as expected:\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":222,"value":"{\n // 12:50am\n let before = spacetime('2020-11-01T00:50:00', 'America/Chicago')\n // 3:10am\n let after = spacetime('2020-11-01T03:10:00', 'America/Chicago')\n return after.since(before).precise\n}","pinned":true,"hidden":false,"mode":"ojs"},{"id":117,"value":"md`\n---\n## Southern hemisphere\nin Australia or Chile, DST changes at a similar time, but in the opposite direction.\n\nFor this reason, we have hard-coded hemisphere information for each IANA timezone (where possible).\n\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":134,"value":"md`\nHere you can see a 'spring-back' DST, where 2am is repeated, in Australia:\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":185,"value":"{\n let justBefore = Object.assign(south, {hour:1, minute:20})\n let d = spacetime(justBefore, 'Australia/Adelaide') // 2am April 5th\n let results = []\n for (let i = 0; i < 9; i += 1){\n d = d.add(20, 'minutes')\n let isDST = d.timezone().current.isDST\n results.push([d.format('nice'), isDST ? 'DST' : 'standard'])\n }\n return html`${results.map(o=>'

'+o[0]+' - '+o[1]+'

')}`\n}","pinned":false,"hidden":false,"mode":"ojs"},{"id":211,"value":"md`Spacetime supports dst-changes in both hemispheres, but will *default to Northern hemisphere* when using an ambiguous timezone, like \\`UTC-5\\`. \n*(It's home to 90% of the earth's population.[¹](https://en.wikipedia.org/wiki/Northern_Hemisphere))*\n\n---\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":72,"value":"html``","pinned":false,"hidden":false,"mode":"ojs"},{"id":299,"value":"parseDST = (dstChange, num) => {\n let fall = dstChange.split('->')[num]\n const [month, rest] = fall.split('/')\n let [day, hour] = rest.split(':')\n return {\n year: new Date().getFullYear(),\n month: Number(month) - 1, //\n date: Number(day),\n hour: Number(hour),\n minute: 0\n }\n}","pinned":false,"hidden":false,"mode":"ojs"},{"id":302,"value":"spring = parseDST(spacetime().timezones['america/chicago'].dst, 0)","pinned":false,"hidden":false,"mode":"ojs"},{"id":309,"value":"fall = parseDST(spacetime().timezones['america/chicago'].dst, 1)","pinned":false,"hidden":false,"mode":"ojs"},{"id":338,"value":"south = parseDST(spacetime().timezones['australia/adelaide'].dst, 0)","pinned":false,"hidden":false,"mode":"ojs"},{"id":56,"value":"spacetime=require('spacetime@latest')","pinned":false,"hidden":false,"mode":"ojs"}],"files":"$W31","stdlib":"1","titling":"auto"}