Problem with datetime in mysql

I have this table with 3 datetime columns. 2 of them are startdate and enddate. I just can't figure out how to insert into those 2 columns. I'm using NOW() for the first one. But startdate and enddate are just 2 hardcoded strings as is now.

mysql_query("INSERT INTO news (ahugaid, titill, stutt, langt, hvenaer, virkt, startdate, enddate) VALUES ('$ahugaid', '$title', '$short_text', '$full_text', NOW(), '$virkt', '$startdate', '$enddate');");

This is how I do it as is and startdate and enddate always end up having the default value. Any solution ?