Description:
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I stumbled over a strange bug. A specific date (1978-05-28) gives a parsing error, even with different patterns. I will append a test script, but you can see it in action here: https://3v4l.org/NDnbm

I used a common italian pattern ("dd/MM/yyyy"), but the issue arise even with the one used by default by Symfony: "yyyy-MM-dd".

Test script:
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<?php

$formatter = new \IntlDateFormatter('it', 2, -1, "Europe/Rome", 1, "dd/MM/yyyy");
$formatter->setLenient(false);

echo 'Pattern 1: dd/MM/yyyy';
echo "\n";
echo "27/05/1978: ";
echo $formatter->parse('27/05/1978');
echo "\n";
echo "28/05/1978: ";
echo $formatter->parse('28/05/1978');
echo "ERROR: ";
echo intl_get_error_message();
echo "\n";
echo "29/05/1978: ";
echo $formatter->parse('29/05/1978');

echo "\n\n";
echo 'Pattern 2: yyyy-MM-dd (Symfony\'s default)';
$formatter = new \IntlDateFormatter('it', 2, -1, "Europe/Rome", 1, "yyyy-MM-dd");
$formatter->setLenient(false);

echo "\n";
echo "28/05/1978: ";
echo $formatter->parse('1978-05-28');
echo "ERROR: ";
echo intl_get_error_message();

Expected result:
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Pattern 1: dd/MM/yyyy
27/05/1978: 265071600
28/05/1978: 265156200
29/05/1978: 265240800

Pattern 2: yyyy-MM-dd (Symfony's default)
28/05/1978: 265156200

Actual result:
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Pattern 1: dd/MM/yyyy
27/05/1978: 265071600
28/05/1978: ERROR: Date parsing failed: U_PARSE_ERROR
29/05/1978: 265240800

Pattern 2: yyyy-MM-dd (Symfony's default)
28/05/1978: ERROR: Date parsing failed: U_PARSE_ERROR