Well that's a good start. Here's a pattern that could be worth investigating. Pseudo-code: for each table t:
1) truncate target.wrk.t
(a work table - truncated every run, same columns/constraints as the main target table t)
2) select @MaxDate = max(dateCreated, DateModified) from target.dbo.t
3) insert target.wrk.t
select [columns] from source.dbo.t
where max(dateCreated, DateModified) >= @MaxDate
4) merge target.wrk.t into target.dbo.t
If you haven't even tried to resolve your issue, please don't expect the hard-working volunteers here to waste their time providing links to answers which you could easily have found yourself.