Under Category click Date select the date format you want and then click OK. Say you want to calculate the 15 discount off of 20. Press Ctrl1 to launch the Format Cells dialog and click the Number tab.
For example 50 20 and 50 -20 are actually the same thing.
Now look at the result if you input your formula. In mathematics subtracting a number from another number is same as summing a positive and a negative number. Type press the equals key to start writing your formula Click on the first cell to be added B2 in this example Type thats the plus sign. Under Category click Date select the date format you want and then click OK.