Date input fields in forms are initially hidden to not clutter the interface. You can reveal them with clicking on "Show" beside the "Date" label.
To manage uncertainty in time up to 4 dates can be used: a timespan for the beginning and a timespan for the ending.
For example if you knew the exact birth of an actor you could enter 1356-12-23 in the first row and add a comment e.g. "It was a good day to be born".
If you are unsure e.g when a Church was destroyed you could use both end dates (chose a wide enough span where you are sure it had happened), enter in the first end year: 1800, in the second end year: 1809.
The application will try to fill missing days or month so you would have created the time span 1800-01-01 to 1809-12-31.
|Year||from -3713 to 9999 but not zero|
|Month||from 1 to 12|
|Day||from 1 to 31|
|Comment||a text field for additional information for begin or end|
Dates will be validated in the backend so you can just try if you aren't sure about a leap year or something like that.
Most limitations come from PostgreSQL (the database) which in turn uses the Proleptic Gregorian calendar. Thats why there is no year zero (one year before year 1 is the year 1 BC).