Trouble creating error message in SQL Server

I have the following piece of code within my stored procedure, I know it doesn't work and should not work but I wanted to illustrate my intentions:

 declare @ErrorMessages varchar;
 set @ErrorMessages = 'An existing deposit on this property ends after the intended start date for the new deposit. ' +
    'Existing End Date: ' + @PreviousDepositEndDate + '. Intended Start Date: ' + @TenancyAgreementStartDate
  raiserror 50002 @ErrorMessages

Can anyone tell me what I should be doing? Or any links on creating this type of string.

EDIT: Forgot to say that the @Dates are both of datetime, the error message is that it cannot be converted from datetime to string

Solution

Here's a slightly different version which some people like because it emulates C printf style:

-- Test data
declare @PreviousDepositEndDate varchar(30) = cast(getdate() - 1 as varchar(30))
    , @TenancyAgreementStartDate varchar(30) = cast(getdate() as varchar(30))
-- Throw
raiserror (N'An existing deposit on this property ends after the intended start date for the new deposit. Existing End Date: %s. Intended Start Date: %s',
           16, -- Severity,
           1, -- State,
           @PreviousDepositEndDate, -- First argument.
           @TenancyAgreementStartDate) -- Second argument.

More info can be found in this MSDN link: http://msdn.microsoft.com/en-us/library/ms178592.aspx