ksh: bad number (with leading zeroes) should not work for 0 - 7

[Abel Abraham Camarillo Ojeda]

Some of our shell scripts that work with dates and do something like:

month=`date +%m`
something && month=$((month-1))

Suddenly started crashing on august... there seems to be a bug identifying
not-numbers (numbers with leading zeroes) before '08' (eigth), how to reproduce:

$ for i in 0{0,1,2,3,4,5,6,7,8,9}; do a=$i; a=$((a-1)); echo $a; done
-1
0
1
2
3
4
5
6
ksh: 08: bad number `08'
$


Thanks.