Index on Datatime field

My version is 11.2.0.2 on solaris 10. Currently the table only has primary key index. The query are mostly using the date field in the where clause which takes long time. I am not sure if it will be a good idea to add index to the datetime field since this is a nullable field. Then I read about create index with nvl function for a string replace with 'null' and for a number replace with '0'. But, it didn't mention about the datetime type. Will create an index helpful in my case? Please advice. Thank you very much.