Do I understand this correctly ?
You have a report which can be ran off monthly ( or for other periods ? ) so you want a prompt when it's scheduled so you can choose "Last Month" or whatever period you want to schedule it for ?
Basically what you need to do is write formula which looks at your prompt, if it says "Last Month" your formula should work out the start and end dates for last month and store them in variables.
You will then need 2 new formulas for the Start Date and End Date which essentially just return the two variables you set in the other formula, these formulas are then used in the Record Selection thing like this:
Record.Date >= {@StartDateFormula} and
Record.Date <= {@EndDateFormula}