You could try a split form (not a big fan) because AFIAK, it's the only way you can get a header along with a datasheet view. In the header you'd put combos/textboxes to filter the datasheet view.
Your details don't indicate a need for a subform, but that is the other way. Same controls in the main form to provide options and then requery the datasheet form based on those options. My guess is that your db design is part of the problem - your timesheet table has no primary key field, or you omitted it from your post. If your db is not properly designed, you are going to continue to have trouble.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.