The temporal datatypes 'TIME/DATETIME/TIMESTAMP' supports fractional values starting from MySQL 5.6.4 version.

See: http://dev.mysql.com/doc/refman/5.6/en/fractional-seconds.html

Without defining the number of fractional decimals it defaults to 0. In this case '2013-04-01 23:59:59.500' rounds to '2013-04-02 00:00:00'.

This affects tests in test suite:
example from:
org.hibernate.test.cid.CompositeIdWithGeneratorTest.testDetachedProperty

// change a non-id property, but do not persist
Date persistedTimestamp = record.getTimestamp();
Date newTimestamp = new Date(persistedTimestamp.getTime() + 1);
record.setTimestamp(newTimestamp);

assertEquals( df.format(persistedTimestamp), df.format(found) );

There is more tests affected by this, I'll put list in comments