How to pass DateTime as varchar in SQL Server 2008?

@MemberType int,
@Period int,
@UnitNo int,
@BranchUserId varchar(100),
@FromDate datetime,
@ToDate datetime

AS

BEGIN

DECLARE @sql AS NVarchar(4000)

    SET @sql = 'select a.userid as [UserID],b.fullname as [Full Name],CONVERT(date,a.regdate,112) as [Joined Date],a.period as [Month Period],a.unitno as [UnitNo],b.address as [Address] from users a join profiles b on a.accountid=b.accountid where (1=1)'
    
    if @FromDate is not null
    set @sql = @sql + 'And a.regdate >= ' + @FromDate + ''
    
    if @ToDate is not null
    set @sql = @sql + 'And a.regdate <= ' + @ToDate + ''
    
    if @MemberType is not null
    set @sql = @sql + ' And a.membertype = ' + @MemberType
    
    if @Period is not null
    set @sql = @sql + ' And a.period = ' + @Period
        
    if @UnitNo is not null
    set @sql = @sql + ' And a.unitno = ' + @UnitNo
    
    if @BranchUserId is not null
    set @sql = @sql + ' And a.branchuserid = ' + @BranchUserId + ''
 
EXEC(@sql)

END