date -d 'last-monday - 14 days' %Y%m%d from 20190315

date -d 'last-monday - 14 days' %Y%m%d from 20180217

date -d 'last-monday - 14 days' %Y%m%d from 201700914

$ date
Wed Mar 20 15:02:23 MST 2019
$ date -d "last-monday"
Mon Mar 18 00:00:00 MST 2019
$ date -d "last-monday - 1 week"
Mon Mar 11 00:00:00 MST 2019

# date
Wed Mar 20 23:02:18 CET 2019
# date -s '20190315' > /dev/null; date -d 'last-monday - 14 days'
Mon Feb 25 00:00:00 CET 2019