Why is Date taking negative values?

Question

By MDN

"Date objects are based on a time value that is milliseconds from January 1, 1970 UTC.

Then why does it take negative values?

Even if these should not negative values ​​mean the values ​​before January 1, 1970?

new Date('0000', '00', '-1'); // "1899-12-30T05:00:00.000Z" new Date('0000', '00', '00'); // "1899-12-31T05:00:00.000Z" new Date('-9999', '99', '99'); // "-009991-07-08T04:00:00.000Z" 

What's happening?

Update

For some positive values, the year begins in 1900.

 new Date(100); // "1970-01-01T00:00:00.100Z" // it says 100Z new Date(0100); // "1970-01-01T00:00:00.064Z" // it says 64Z new Date("0006","06","06"); // "1906-07-06T04:00:00.000Z" 

Also note that in the latter case, the date is displayed as 4, which is incorrect.

I suspect this is some kind of Y2K error ?! !!

Answer 1

It's complicated and inconsistent, yes. The JavaScript Date object was based in Java 1.0, which is so bad that Java has redesigned a completely new package. JavaScript is not so lucky.

• Date is based on unix eopch because it is defined . Internal details.
• 1st Jan 1970 is the actual time of this baseline.
• since is the direction of the timestamp value: forward for + ve, backward for -ve.
• Externally, the Date constructor has several different ways of using based on parameters:

Zero parameters = current time

 new Date() // Current datetime. Every tutorial should teach this. 

The time is absolute, but the “displayed” time zone can be UTC or local.

For simplicity, only UTC will be used in this answer. Keep time zone when testing.

One numeric parameter = timestamp @ 1970

 new Date(0) // 0ms from 1970-01-01T00:00:00Z. new Date(100) // 100ms from 1970 baseline. new Date(-10) // -10ms from 1970 baseline. 

One string parameter = iso date string

 new Date('000') // Short years are invalid, need at least four digits. new Date('0000') // 0000-01-01. Valid because there are four digits. new Date('1900') // 1900-01-01. new Date('1900-01-01') // Same as above. new Date('1900-01-01T00:00:00') // Same as above. new Date('-000001') // 2 BC, see below. Yes you need all those zeros. 

Two or more parameters = year, month, etc. @ 1900 or 0000

 new Date(0,0) // 1900-01-01T00:00:00Z. new Date(0,0,1) // Same as above. Date is 1 based. new Date(0,0,0) // 1 day before 1900 = 1899-12-31. new Date(0,-1) // 1 month before 1900 = 1899-12-01. new Date(0,-1,0) // 1 month and 1 day before 1900 = 1899-11-30. new Date(0,-1,-1) // 1 month and *2* days before 1900 = 1899-11-29. new Date('0','1') // 1900-02-01. Two+ params always cast to year and month. new Date(100,0) // 0100-01-01. Year > 99 use year 0 not 1900. new Date(1900,0) // 1900-01-01. Same as new Date(0,0). So intuitive! 

Negative Year = BC

 new Date(-1,0) // 1 year before 0000-01-01 = 1 year before 1 BC = 2 BC. new Date(-1,0,-1) // 2 days before 2 BC. Fun, yes? I'll leave this as an exercise. 

No 0 AC. There is 1 AC and a year up to 1 BC. Year 0 by agreement 1 BC.

2 BC is displayed as the year "-000001" . Extra zeros are required because it is outside the normal range (0000 to 9999). If you are new Date(12345,0) , you will get "+012345-01-01" too.

Of course, the Gregorian calendar , adopted back in 1923 in Europe, will cease to be significant long before we reach BC. In fact, scholars accept that Jesus was not born in 1 BC. But with the stars and Earth moving along this scale path, the calendar is the least of your worries.

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The remaining code is just a variation of these cases. For instance:

 new Date(0100) // One number = epoch. 0100 (octal) = 64ms since 1970 new Date('0100') // One string = iso = 0100-01-01. new Date(-9999, 99, 99) // 9999 years before BC 1 and then add 99 months and 98 days 

I hope you enjoyed it. Please do not forget to vote. :)

To keep working, save all dates in ISO 8601 and use the string constructor.
And if you need to process the time zone, save all dates in UTC.

Answer 2

Well, first of all, you pass a string instead of an integer, so this may have something to do with your problems here.

Check this option , it explains the negative dates pretty nicely, and there is an explanation for your exact example.

Answer 3

Please see the documentation.

Year: Values ​​from 0 to 99 cards for the years 1900 to 1999.

• 1970 with the appropriate time zone: new date (0); // int MS since 1970
• 1900 (or 1899 with an applied time zone): new Date (0,0) or new date (0,0,1) - date 1 based, month and year 0 based
• 1899: new date (0,0, -1)

Answer 4

Then why does it take negative values?

You are misleading to describe how data is stored internally with arguments that the constructor function performs.

Even if these should not negative values ​​mean the values ​​before January 1, 1970?

No, for that reason. Nothing stops a year, month or day from denial . You just add a negative number to something.

Also note that in the latter case, the date is displayed as 4, which is incorrect.

Numbers starting with 0 are expressed in octal , not decimal. 0100 === 64 .