let's say I have a combo box with value one month, two month and three month. When the user select one month, it means that the user want to see all results a month ago. Eg, if today is 25-Oct. When he selects one month, then all result from 25-Sep to 25-Oct. For now, just forget about the special case like Feb or any special case first. Which date function is best apply for this case?