Hi guy when i run this Stored procedure it give me message error

conversion faild when converting date time from charachter sting

stored procedure as following

Create proc searchData
    @StartDate datetime, 
    @EndDate datetime
as
Declare @SQLQuery as nvarchar(2000)
SET @SQLQuery ='SELECT * from Employee Where (1=1)' 
  If (@StartDate is not NULL) AND (@EndDate is not NULL)
         Set @SQLQuery = @SQLQuery + ' And (JoinDate 
         BETWEEN '+ @StartDate +' AND '+@EndDate+')'
Exec (@SQLQuery)

JoinDate found in table Employee as datetime

but when i make stored procedure as following

it work in formate dd/mm/yyyy and this is what i need 

ALTER  proc [dbo].[searchData]
@StartDate datetime 
@EndDate  datetime, 
as
select * from dbo.Employee e where JoinDate between @StartDate and @EndDate

Now what is the proplem in first stored procedure
Please help me if possible