"I have a concern about payroll."
Any carry-over liabilities are Just liabilities and not part of the new year's payroll.
"Our first payroll in 2018 will cover two weeks starting 12/23/17."
Pay Date drives payroll; you are asking about Pay Period.
"Will I be able to upload the timesheets for 12/23/17 thru 12/29/17 into the new file if I use 1/1/2018 as the new start date?"
Yes, for Desktop QB. It never locks you out by "start date" at all.
