Consider a Day class that models a calendar day such as "February $28,1900".$ A difficulty is that sometimes February has $28$ days, and sometimes $29($when the year is a "leap year"$).$

A tester program contains these lines of code:

Day someDay $=$ new Day$(1900,2,28)$; Day nextDay $=$ someDay.addDays$(1)$; System.out.println$($nextDay$.$getMonth$())$; System.out.println$("$Expected: $3")$;

The output is

$2$ Expected: $3$

What does that mean?