Sage 50's system date does not affect your computer's date or the dates in any other applications you are running.
OR click the System Date button on the Button Control Bar that appears at the top of the main Sage 50 window.
If the system date is within the current accounting period, Sage 50 uses this date. If you are not working in the current accounting period, Sage 50 defaults transaction dates to the first day of the current accounting period.