support for generating decreasing dates in bash 4.0 and 4.1

support for generating decreasing dates in bash 4.0 and 4.1

The printf builtin "%(fmt)T" specifier (which allows time values
to use strftime-like formatting) is introduced in bash 4.2.

Trying to execute this in pre-4.2 bash will fail -- and if this
happens execute the fallback piece of perl code to do the same thing.