Description:
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When formatting dates around the year ending IntlDateFormatter produces wrong outputs. Not just unexpected for the locale, but wrong values. More precisely: it is off by a year. This was already reported in #73405.


A. Formatting 2017-12-31 12:00:00. In this case en_US is wrong, off by a year.

Using: en_US, Europe/London, "Y".
- Result: 2018. Expected: 2017. 3v4l: https://3v4l.org/mIUue.

Using: en_US, Europe/London, "Y-MM-dd".
- Result: 2018-12-31. Expected: 2017-12-31. 3v4l: https://3v4l.org/sMO9K.

Using: nl_NL, Europe/Amsterdam, "Y".
- Result: 2017. Expected: 2017. 3v4l: https://3v4l.org/6TJ4R.

Using: nl_NL, Europe/London, "Y".
- Result: 2017. Expected: 2017. 3v4l: https://3v4l.org/NdjNM.


B. Formatting 2017-01-01 12:00:00. In this case nl_NL is wrong, off by a year.

Using: nl_NL, Europe/Amsterdam, "Y-MM-dd".
- Result: 2016-01-01. Expected: 2017-01-01. 3v4l: https://3v4l.org/oeWEb.

Using: en_US, Europe/London, "Y-MM-dd".
- Result: 2017-01-01. Expected: 2017-01-01. 3v4l: https://3v4l.org/7ibRK.

Test script:
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<?php

$formatter = new IntlDateFormatter(
    'en_US',
    IntlDateFormatter::FULL,
    IntlDateFormatter::FULL,
    'Europe/London',
    IntlDateFormatter::GREGORIAN,
    "Y-MM-dd"
);


echo $formatter->format(
    new DateTime('2017-12-31 12:00:00', new DateTimeZone('Europe/London'))
);

Expected result:
----------------
2017-12-31

Actual result:
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2018-12-31