By default use "Last Menstrual Period": pick the first day of your LMP and the due date is calculated as LMP + 280 days (40 weeks, Naegele rule). Or switch to "Due Date" and enter it directly. The page shows days remaining, current week (Xw + Yd), stage and a progress bar.
The common formula is LMP + 280 days (40 weeks). Early ultrasound estimates usually take precedence. Results are for reference; follow your doctor's advice.