DAYS OF THE MONTH

[Max Power]

Hi,
o.s.: OpenBSD 5.3/amd64

If I create a directory with the command: mkdir $(date +'%d')

why this is the result: 1, 2, 3, 4, 5, 6, 7, 08, 09, 10, etc.
Why the '0' [zero] appears only ahead the digit 8 and 9..?

Thanks.