Is it a good idea to index datetime field in mysql?

I am working on designing a large database. In my application I will have many rows for example I currently have one table with 4 million records. Most of my queries use datetime clause to select data. Is it a good idea to index datetime fields in mysql database? Select field1, field2,.....,field15 from table where field 20 between now() and now + 30 days I am trying to keep my database working good and queries being run smoothly More, what idea do you think I should have to create a high efficiency database?