Why date in tray resets?

I can understand your pain of resetting date of your computer, every time you start your computer. As you have told us you have an old desktop computer, the problem exists in motherboard`s battery. You need to replace the battery of the motherboard. Replacing the battery is very simple and easy. Open the C.P.U. of the computer and you will see the green board, which is the motherboard. On the motherboard you will find a round battery (a glossy one). Take off the battery, it is like removing the battery from the watch. Then you need to install a new lithium battery of 3volt. This battery will cost you $3-5 dollars. When you've bought the battery, just fix the battery in the motherboard in place of old battery. Don`t hold the battery with your hands because it has static power. Make use of a cloth. After fixing the battery, close the C.P.U, start you computer and change the date.