Write a user-defined function that calculates the average collection period with two arguments: accounts receivable, and annual credit sales, and assumes that there are 360 days per year.

Now, write another version of the function that accepts an optional third argument that indicates whether to use 360 or 365 days in a year. This third argument should default to using a 360-day year. The second function should return an error value (look up CVErr in the help) if the user specifies any number other than 360 or 365 days per year.