0 (zero)
Because roman numerals have no symbol for 0, you start on a clock with Ⅻ. With arabic numerals it could be otherwise.
Explanation
An analoge clock can just as well start with 0.
Ⅻ ⅩⅠ Ⅰ Ⅹ ⅠⅠ Ⅸ Ⅲ Ⅷ ⅠⅠⅠⅠ Ⅶ Ⅵ Ⅴ
0 11 1 10 2 9 3 8 4 7 6 5 Roman Arabic
After all, on a digital clock you get
00:00
with double zero, and that surprises apparently nobody.