Maintaining Variable After EXEC

Hello,

I am fairly new at stored procedures. I have created some that will
go through a table and return a start date and an end date that is
dependent upon the fiscal period you want, but I then need to use
those dates in another stored procedure to retrieve the information I
need. My stored procedure looks like this.

================================================== ====================

CREATE PROCEDURE dbo.R920ExtTotal
@MthsBack Decimal OUTPUT
AS

DECLARE @sSQL AS NVARCHAR(255), @StartDate as SMALLDATETIME, @EndDate
as SMALLDATETIME

Exec @StartDate = GetMthStart @MthsBack

Exec @EndDate = GetMthEnd @MthsBack

SET @sSQL = 'Select count(extension) as Total From r920f00 Where
([date] BETWEEN "' +
CONVERT(nvarchar, @StartDate) +
'" and "' +
CONVERT(nvarchar, @EndDate) +
'")'

Select @sSQL

EXEC (@sSQL)

Return
GO

================================================== ===============

The problem is my variables @StartDate and @EndDate do not retain
their values after the EXEC statement and revert to 01/01/1900. How
can I get around this problem?

Thanks!!!!

Chip