Nothing to do with POSIX. you just need to work out how to get the correct date out of localtime.

use constant DAY => 60 * 60 * 24;
use POSIX 'strftime';

print strftime('%Y%m%d', localtime(time - (14 * DAY)));
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"The first rule of Perl club is you do not talk about Perl club."
-- Chip Salzenberg