Basically system date will be taken as sales order date. If you want your order to be placed on the date you like, then you have to change the session date (Tools → Session date and time).
If it is related to sales delivery, then you have an option of changing the delivery dates by going to sales order (header view → delivery tab)
Like Lixin said, you can do the process for changing the date.
One more thing,When you give the price & dates (To date and From date) in the Trade agreements then it will fetch the same price between the dates which you mentioned in the trade agreement forms and same price will be considered & it ignore the earlier price. check your date ranges mentioned in TA.
For ex: if your selecting Trade agreements > Sales Price. In the Sales price form mention Quantity,price and to/ from dates then it will fetch the same price & if you mention any discounts show it in the sale order lines> price/discount tab.
If you did all the above mentioned setups properly and still your facing problem then let us know what error it is throwing.