Calendar.roll weird result when using Calendar.DATE

Question

Today, in Europe, the date is 1 October 2015. Can you tell me why running this code displays the date as 31 Oct 2015?

Calendar yesterday = Calendar.getInstance();
yesterday.roll(Calendar.DATE, false);
System.out.println(yesterday.getTime());

Answer 1

Calendar.DATE represents the day of the month, not the day of the year. Thus, when rolling the day of the month backwards of 1 unit, we are essentially going from the 1st to the last day of the month.

Quoting the roll Javadoc (emphasis mine):

Adds or subtracts (up/down) a single unit of time on the given time field without changing larger fields .

and quoting the Calendar Javadoc:

A larger field represents a larger unit of time.

As such, fields like MONTH or YEAR are not changed by this method when rolling the day of the month.

You should use Calendar.DAY_OF_YEAR instead:

Calendar yesterday = Calendar.getInstance();
yesterday.roll(Calendar.DAY_OF_YEAR, false);
System.out.println(yesterday.getTime());