Problem adding new birthday, date not accepted

When I want to add an new birthday in the list, some dates are ok for the plugin and some are not, e.g. I just updated the list with a new collegue who has her birthday on August 18th. No problem to add that new entry, writing 18. August 2017. Another new collegue has his birthday on February, 2nd. The system does not accept this data and shows the following entry: 30. November -0001. If I select 18. August 2017 or 18. September 2017 instead, everything is fine. But the same mistake occurs for birthday dates like 16. Mai 2017 or 31. January 2017. Help is really appreciated. Thanks!