Variable in Compare Object input in Powershell

Please excuse my terminology as I'm a powershell/scripting novice, however I have a script developed that compares two csv files and provides differences between them in an output csv file.  I have another script that is used to execute the exchange powershell query to gather the date.  I have another question open on how to get those results in a date variable.  I would need to know how I can have the compare script, which I have included a piece of below, change the compare-objects each day it runs.

The data gather script is set to run once per day, dumping the results in the same folder location as this script.  I want to change the Export.csv and Exportold.csv to varibles based on date, so every time the script runs, it will change the variable of each of htese properties to one name being the current date, and comparing it to yesterday's date.

For example:

export.csv would be 20131223calpermissions.csv
exportold.csv would be 20131222calpermissions.csv

And this would automatically change each day, without having to modify the script.


compare-object (import-csv Export.CSV | sort) (import-csv Exportold.CSV | sort) -passthru -property Mailbox,User,'$_.AccessRights',IsValid |

If there is a better way of doing this, i'm open to suggestions, and again I'm a script novice so bear with me.  Thank you in advance!