You initialized GregorianCalendar with default locale and time zone, but Codeforces uses time zone with 23- or 25-hour days at some dates due to daylight savings. Because of that, your difference in milliseconds sometimes gets minus one hour becoming minus one after integer divisions.

Your accepted solution, in contrast to others, uses rounding, thus unintentionally leading to correct answer. As for your first submission, it uses default constructor, which initializes GregorianCalendar with current date and time. When you set year, month and day, GregorianCalendar tries to adjust time of the day in some complicated way according to time zone, resulting in one more hour lost between 1900:01:01 and 2038:12:31. This doesn't happen in your second submission, because it doesn't use default constructor.