Trying to use the GregorianCalendar, I got stuck on a singularity while computing the number of days since a particular date. In the scala interpreter, I entered :
scala>import java.util.GregorianCalendar
scala>import java.util.Calendar
scala>val dateToday = new GregorianCalendar(2012,Calendar.MAY,22).getTimeInMillis()
dateToday: Long = 1337637600000
scala>val days1 = (dateToday - (new GregorianCalendar(1976,Calendar.MARCH,28).getTimeInMillis())) / (1000*3600*24)
days1: Long = 13203
scala>val days2 = (dateToday - (new GregorianCalendar(1976,Calendar.MARCH,29).getTimeInMillis())) / (1000*3600*24)
days2: Long = 13203
I don't know if the fact that 1976 is a leap year matters, but days1 and days2 should have been separated by 1. This is the only moment in history since 1970 that this singularity happens.
Wanting to know what is going on, I compute the difference between the two dates previously mentionned, and it gives me only exactly 23 hours of difference ! What happened on that date ? Wikipedia apparently says nothing about it.
And even more important, how to compute the real number of days since a particular date ?
If the option of a float division is not available, the best answer I can think of is to add one hour (1000*3600) while computing a difference between two days:
scala> (dateToday - (new GregorianCalendar(1976, Calendar.MARCH, 28).getTimeInMillis()) + 1000*3600) / (1000 * 3600 * 24)
days1: Long = 13204
scala> (dateToday - (new GregorianCalendar(1976, Calendar.MARCH, 29).getTimeInMillis()) + 1000*3600) / (1000 * 3600 * 24)
days1: Long = 13203
It should work for every date, as you do not suffer from leap hours anymore.