Leap years are each fourth year, except each hundredth year, except each thousandth fourhundredth year.
1896 leap year
1900 not leap year
1904 leap year
...
1996 leap year
2000 leap year
2004 leap year
...
2096 leap year
2100 not leap year
2104 leap year
Then you just arrange the 10 year window in different positions to overlap 1 to 3 leap years to reveal the three outcomes of the bug.
- / - - - / - - - /
- - / - - - / - - -
- - 0 - - - / - - -
- is a normal year, / is a leap year, 0 is an exceptional non-leap year.