how to get information about discounts on product?
On the line item, you have the field_unit_price field. Inside of that field's data attribute, you'll find information about the discounts that are applied to that product:

array(1) {
  ["components"]=&gt;
  array(2) {
    ...
    [1]=&gt;
    array(3) {
      ["name"]=&gt;
      string(22) "discount|discount_test"
      ["price"]=&gt;
      array(3) {
        ["amount"]=&gt;
        float(-240)
        ["currency_code"]=&gt;
        string(3) "USD"
        ["data"]=&gt;
        array(2) {
          ["discount_name"]=&gt;
          string(13) "discount_test"
          ["discount_component_title"]=&gt;
          string(5) "Test"
        }
      }
      ["included"]=&gt;
      bool(true)
    }
  }
}


Iterate over the components. If there is a discount applied to one of the items, you'll find one with a name that begins with discount|.  Then you can then load that discount and grab more information about it:

$discount = entity_load('commerce_discount', array('discount_test'));


From here, if you've added dates, the date fields will be accessible on the field commerce_discount_date.