Parameters are a way for the user to input a single value into tableau - but that said they don't do anything by themselves - they have to be backed up with a calculation or a filter
So if you would need to know 2 things - start and end date or start date and duration - that would be 2 parameters if the range (duration) is variable or only 1 if it is a constant number of months
then you would write a formula like Month(date) >=Month(start date) and month(date) <= month(end date) it will result in a t/f filter that you can set to True
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