I really like this Condition, because it makes it so easy to find people that joined anytime during an entire month when you specify how many months to look back. The Condition is located on the Membership category tab and is named simply JoinDateMonthsAgo.
One of the best use cases for this is to find people that are approaching their 1 year anniversary as a church member. Use 11 as the number of months to look back and you will find everyone whose 1 year anniversary is going to be in the next month. Save the search and run it each month. You can send the people a note acknowledging the anniversary. However, you can also encourage them to get more involved and volunteer. Most churches require that a person be a member for a year before they can teach a class or volunteer for other positions within the church. This is a great Condition to use to find those folks.