How to Add or Subtract Hours from a Date

This code example shows how to add and remove hours from a date using an instance of the Calendar class.
The method incrementHours takes two parameters, the Calendar object and the number of hours to add.
The method decrementHours takes the same parameters and decrement the Calendar object with the number of hours passed to it.
Note that the actuall call to the calendar is to the same method, add(), but when decrementing a negative value is passed.
Also note that in the example we use the constant HOURS of the Calendar class. That constant is used when dealing with a 12 hour clock.
If we utilize the 24 hour clock we should instead use the constant HOUR_OF_DAY.

The main method creates a Calendar object and print out the current date / time.
Then it instantiates a new Main object and call both methods with the calendar instance as parameter along with the number of hours.

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