Inserting dates can be tricky, since it looks as if thing are okay when you are using strings. You have to use the format: yyyy-MM-dd HH:mm:ss (you can leave out the time-part AFAIK).
insert into musicians (name, inactivedate) values ('David Bowie', '2016-01-10');
insert into musicians (name, inactivedate) values ('Prince Rogers Nelson', '2016-04-21');
insert into musicians (name, inactivedate) values ('Elvis Presley', null);