Dates in Genney are entered by default in the format that belongs to the computer's language selection and in Swedish, which is also the international standard, it is ISO 8601, i.e. yyyy-mm-dd. If you want to change it, there are 21 other date formats to choose from under settings > date in Genney.
The date format selected applies to both input and presentation. Date entry in Genney can also always be entered in gedcom's date format, for example BEF 19 OCT 1789, and it is then converted to the current format when exiting the field's edit mode.
Entering dates in Genney is the responsibility of the user to use the correct syntax. Next to the date fields there is a help box to facilitate how to enter the variants of formats that exist under the gedcom standard. Valid date formats are, for example, before 1789-02-12 or about 1789. What is put between brackets after the prefix in the help box are short variants. If you write < 1789, it is converted to before 1789, and if you write 1789 - 1822, it is converted to from 1789 to 1822, etc.
It is also possible to enter an invalid date format, but it is then interpreted as text, which means that it is not sortable. Dates with invalid date format are marked in Genney with a red spiky underline. Even dates that are not in the calendar, as in the image example, or dates that are unreasonable are marked and the reason for the marking is shown with a tooltip when you hover the mouse over the date.
Under settings > control, it is possible to change different values for when Genney should warn and/or to turn off the date control.
It has easily happened that you slipped on the key or that an import from another genealogy program did not export the dates correctly. To find errors in dates, there is a check function in Genney under the tools menu > maintenance > incorrect dates. There are also other checks for dates for people born after the death of the mother, etc.
Read more here.