Javascript getFullYear（）日期方法奇怪的行为

Question

Can anyone explain why getFullYear does not return 2014?

console.log(new Date('2014-01-01').getFullYear()) //2013
console.log(new Date('2014-01-01').getUTCFullYear()) //2014

Answer 1

From MDN :

The dateString of "March 7, 2014" returns a different date than "2014-03-07" unless the local time-zone is UTC. When converting a dateString of "March 7, 2014" the local time-zone is assumed. When converting a dateString of "2014-03-07" the UTC time-zone is assumed. This results in two different Date values depending on the format of the string that is being converted.

So when you ask it to parse "2014-01-01", you're getting the time in UTC.

Then you call .getFullYear() on your object, which uses local time. If you live in the Eastern US like I do, then it basically subtracts 4 hours from the internal time and returns the year.

So here's what happens:

1. "2014-01-01" is converted to "1388534400000"
2. .getFullYear() is called and "1388534400000" is converted to local time
3. Local time is something like "1388534160000"
4. New years hasn't yet occurred at "1388534160000", so it's still 2013

All of this implies that if we do something like

console.log(new Date('January 1, 2014').getUTCFullYear()); // 2014
console.log(new Date('January 1, 2014').getFullYear()); // 2014

We'll get the same year, because we told the browser to use our timezone right on New Year's, but they're not equivalent:

console.log(new Date('January 1, 2014').getUTCHours()); // 5
console.log(new Date('January 1, 2014').getHours()); // 0

Answer 2

According to this : "The difference is when you specify a string in the format YYYY-MM-DD, you get a date that is 12am in the GMT timezone and when you specify a date in the format DD-MM-YYYY, you get a date that is 12am in your current timezone."

So basically since you are specifying News Years Day 2014 when it gets converted from GMT to your local time it believes it is 12-31-13 not 01-01-14.