Fetch discount start date and end date
I don't understand why do you need that..

From your question I can assume you have both the discount module and coupons module..

If using discount in general, you don't need to apply a coupon, and the discount will be applied with rules only on the dates that you put in on the default discount creation form.

The same goes for coupon creation, when creating a coupon, you can set coupon criteria as date range and that will also be applied by default in coupon validation.