How to add Limit to Date_Time in SQLite for Android

The following line is used for query in Android, where orderBy is the parameter

String orderBy = Helper.COLUMN_DATE_TIME +" DESC";

This arranges the table rows in descending order of time. The value of date_time is in the format %Y-%m-%d %H:%M:%S

I would like to introduce LIMIT in the above statement (as in some instances I am only in need of %Y-%m-%d). How can this be done? I mean I am limited by my knowledge and try. Any help would be appreciated