Error when using the Concat function in custom field formula

I am trying to combine the value of the following fields: Company; Last Name; a custom pick list field; and a custom date field into a 5th resultant field.

All goes well when using the Concat function and the first 3 fields however, when I add the date field and then I receive the error "Incorrect Data Type of Argument" . Of course this makes sense but I can't find a way around it.

The formula I am using is that works is:

Concat(${Leads.Company},',',' ',${Leads.Last Name},',',' ',${Leads.Event Type of Event})

An example of the result is; ABC CORPORATION, HUTSON, Corporate

The formula I try that does not work is:

Concat(${Leads.Company},',',' ',${Leads.Last Name},',',' ',${Leads.Event Type of Event},',',' ',${Leads.Event Date of Event}))

An example of the result we would like is: ABC CORPORATION, HUTSON, Corporate, 6/21/2011

Is there a solution to this problem.

Thanks in advance for any advice offered,