Auto insert date into date field after clicking button, Javascript

General information for the website: Energy Prices
Kind of development: Customization of existing website
Description of requirements/features: We require a set of dynamic date periods that can be inserted into a date field after clicking a button.

The date ranges would need to be dynamic. As a starting point we would need:
Last week
Last 2 weeks
Last month
Last 6 months
Last 12 months

You can see on our website that we from and to date fields. When a date is set the graph is automatically updated.
http://www.energycentric.universal-web-design.co.uk/energy-market-report-electricity/

We just need to let users click a button for a date period, last 12 months for example and for it to fill in the date range automatically so it updates the graph.

As a starting point the following jsfiddle might be useful:
http://jsfiddle.net/lopac1029/RuhWj/3/
Extra notes: