We are working to fix this issue in a future version, but the current solution is below.
Problem
The back buttons and first row of the popup calendar is not visible on some iPhones when the Zoom Mode is enabled.
Solution
Solution 1
Select the date field a second time and enter the date manually. Please use the format as setup in your user preference MM/DD/YYYY or DD/MM/YYYY
Solution 2
Go to your iPhones setting and disable the Zoom Mode.