Java Date API

java.util.
Class Date
java.lang.Object
extended by java.util.Date
All Implemented Interfaces:
Cloneable, Comparable, Serializable
Direct Known Subclasses:
Date, Time, Timestamp
public class Date extends Object
implements Serializable, Cloneable, Comparable
The class Date represents a specific instant with millisecond precision.

See here for more resources.

Some examples of using it:

Converting Date to stringConverting string to DateDate arithematic
Calculating difference between 2 dates
Getting last day of month