How can I calculate the date preceding a given date in unix?

Question

I have two variables: X and Y.

The value of X will be a date given in the format mmddyy and I want to calculate the date preceding that date

Answer 1

I like Tcl for date arithmetic, even though it's clunky for shell one-liners. Using Tcl 8.5:

x=091509
y=$(printf 'puts [clock format [clock add [clock scan "%s" -format "%%m%%d%%y"] -1 day] -format "%%Y%%m%%d"]' "$x" | tclsh)