Float will pull the expected invoice/bill date from your accounting software. If no date has been set, the due date will be used. If the invoice/bill is not settled by the expected date, Float will roll it forward to show it due for the current day, until it is paid or a new expected date set.
You can change expected dates directly in Float, however if you're a Xero user, and a different expected date is added in Xero, this will overwrite the changes made in Float.
Changing expected dates in Float does not change the expected date in the accounting platform, as the sync is one-way only.