Storing a time interval

I need to create a table to store terms and conditions for purchase
orders.

Some of the attributes of a PO include payment terms. Quite often these will
be 2 periods associated with these, the first is a period on which if you
pay, you receive a discount, and the 2nd is when payment is due with no
discount. I am thinking of storing these two attributes as time internals.

What is the wisdom as to how to declare the type of these columns?


-- 
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
                        -- Benjamin Franklin