Bash: compute dates

Often I found myself in the need of computing some math on dates. For example what is 10 weeks from a given date? Rather than opening a calendar and start counting you can quickly open a bash shell.

Example: what’s 6th of June 2017 plus 10 weeks?

$ date -j -v '+10w' -f '%Y-%m-%d' '2017-06-06'
Tue 15 Aug 2017 10:00:20 BST