Some links:

1934-08-02 works with lang en but not with lang ko
https://www.wikidata.org/wiki/Special:ApiSandbox?setlang=en#action=wbparsevalue&format=json&parser=time&values=1934-08-02
https://www.wikidata.org/wiki/Special:ApiSandbox?setlang=ko#action=wbparsevalue&format=json&parser=time&values=1934-08-02

1773-12-16 works correctly in en but not in ko
https://www.wikidata.org/wiki/Special:ApiSandbox?setlang=en#action=wbparsevalue&format=json&parser=time&values=1773-12-16
https://www.wikidata.org/wiki/Special:ApiSandbox?setlang=ko#action=wbparsevalue&format=json&parser=time&values=1773-12-16

@revi what are you entering and on what item?

@Addshore On 3rd sandbox, "Birth date" property and date: 1999-12-31.

Now I tried to add P571 as 2011-04-19 (19th Apri, 2011) to this item, and result says "2004-11-20 (20th November, 2004)". Diff When I appened ?uselang=en on URL, it worked well, Diff

Note that "2011-04-19" is not a valid ISO timestamp our current parser setup can recognize as an ISO timestamp. It must be "+2011-04-19T00:00:00Z" to be recognized as such.

"2011-04-19" is recognized as, well, something very odd by PHP's build in DateTime parser (see http://php.net/manual/en/datetime.construct.php). We are using this as a fallback. There is nothing we can do to "fix" that part of PHP other than implement our own parsers.

I uploaded https://github.com/DataValues/Time/pull/27. It rewrites our existing parser in a way it can recognize incomplete ISO date strings. Note that strings like "2011-4-19" will still be invalid with this patch.

2011-4-19 is clearly the 19th of April 2011, so why not parse this too?

In T85324#996337, @Addshore wrote:

2011-4-19 is clearly the 19th of April 2011, so why not parse this too?

Yes, 2014-4-19 is pretty clearly the 19th of April 2011. But the string "2011-4-19" does not follow any standard, it is just a format invented by some use on some occasion. So at some undefined point in the past, the meaning becomes unclear. The new parser would have to contain some date to divide between clear dates, which would be accepted, and unclear dates, which would be rejected. What should that date be? Perhaps 1 March 1923, which is, I believe, the last date a country changed from the combination of the Julian calendar and AD/BC year notation to the Gregorian calendar and AD/BC year notation. Other countries have changed since, but the preceding calendar either wasn't Julian or didn't use AD/BC year notation.