I needed to whip up a date delta (in my case, today minus 13 months) for SQL. I couldn't push the calculation onto the database, because the field in question is of type text rather than date, and converting it to a date and then subtracting the delta proved to be too expensive. Creating the date in Perl and using that is far more efficient. Don't blame me, I didn't design the schema, I just use it.
It's perhaps too-clever-by-half in that it's the most-commented bit of code I've written in some time, but I like the way it plays with list context.
use Date::Calc 'Add_Delta_YM';
my $date_limit = sprintf( '%02d-%02d-%04d',
reverse # Y M D --> D M Y
$_ + 1900, # year
$_ + 1, # month
$_, # and day
0, # less this many years
-13, # and this many months
}->(localtime), # from today
How about: SELECT SUBDATE(datefieldname, INTERVAL 13 MONTH);
You can then format the result as needed with DATE_FORMAT(). I'm willing to bet both processes will be more efficient done through database than Perl.
EDIT: Oops - as pointed out by grinder in PM, he already specified that the database approach wouldn't work, since the field is TEXT. Somehow I missed that. Whoever designed the table needs some serious punishment.