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.