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JS Dates Notes

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JS Dates Notes

Terms

  • GMT: Greenwich Mean Time
  • UTC: Coordinated Universal Time
  • GMT ≈ UTC (≒)
  • CST: Central Standard Time

Format: ISO Date

  • YYYY-MM-DDTHH:MM:SSZ
  • YYYY-MM-DD
  • YYYY-MM
  • YYYY
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// Now
Date.now() // 1526128945300
// Now
new Date() // Sat May 12 2018 20:37:57 GMT+0800 (CST)
// The ISO 8601 syntax (YYYY-MM-DD)
new Date('2016-11-18') // Fri Nov 18 2016 08:00:00 GMT+0800 (CST)
new Date('2019-05-18') // Sat May 18 2019 08:00:00 GMT+0800 (CST)
// input：「12:37:57(+0000)」 return：「20:37:57(+0800)」
new Date('2018-05-12T12:37:57Z') // Sat May 12 2018 20:37:57 GMT+0800 (CST)
// input：「21:37:57(+0900)」 return：「20:37:57(+0800)」
new Date('2018-05-12T21:37:57+09:00') // Sat May 12 2018 20:37:57 GMT+0800 (CST)
new Date('2018-05-12T12:37:57Z').toDateString() // "Sat May 12 2018"
new Date('2018-05-12T12:37:57Z').toUTCString() // "Sat, 12 May 2018 12:37:57 GMT"

Substract two dates

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var a = new Date('2016-11-18')
var b = new Date('2019-05-18')
// same
var diff = Math.abs(b - a); // ms
var diff = Math.abs(b.getTime() - a.getTime()); // ms
// get diff days
var diffDays = Math.ceil(diff / (1000 * 3600 * 24)); 
// one line: get days
(new Date('2019-11-02') - new Date('2019-08-17'))/1000/60/60/24
// one line: get months (approximately equal)
(new Date('2019-05-18') - new Date('2016-11-18'))/1000/60/60/24/30.42

Add/Substract days

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var today = new Date()
today.setDate(today.getDate() - 365)
console.log(today)