JavaFAQ Home » General Java
Practical info about Java Calendar and Date
If you want to operate with different dates in your Java program you will use Java Calendar class.
I will try to give you some overview of not widely known facts about Java Calendar and Date classes, working code examples, which you can try right away.
The basic information about Calendar class is provided by Java API. The Calendar class is about days, months and years. One could ask: is not Date class about the same? Not exactly...
What is difference between Java Date and Calendar classes?
The difference between Date and Calendar is that Date class operates with specific instant in time and Calendar operates with difference between two dates. The Calendar class gives you possibility for converting between a specific instant in time and a set of
calendar fields such as
DAY_OF_MONTH. You can also manipulate with the calendar fields, for example getting the date of your grandmother birthday .
I would like to point some things about Calendar and Date which you should know and which are not obvious...
Years, months, dates and hours are in "normal" range like:
- A year y
- A month from 0 to 11
- A date (day of month) from 1 to 31 in the usual manner.
- An hour 0 to 23.
- A minute from 0 to 59 in the usual manner.
But, attention!! A second is represented by an integer from 0 to 61. Looks strange - 61 second, but do not forget about leap second. About once every year or two there is an extra second, called a "leap second." The leap second is always added as the last second of the day, and always on December 31 or June 30. For example, the last minute of the year 1995 was 61 seconds long, thanks to an added leap second.
Another funny feature is lenient and non-lenient fields in calendar. What is that? Example:
32 January 2006. Actually if you set your calendar lenient it will be 1 February 2006 and no problem for your program . If it is non-lenient ArrayIndexOutOfBoundsException exception will be thrown.
Another question is 00:00 end or beginning of day? Is 00:00 A.M. or P.M.? Are midnight and noon A.M. or P.M?
Answer: 23:59 is the last minute of the day and 00:00 is the first minute of the next day. Midnight belongs to "am", and noon belongs to "pm", so on the same day, 12:00 am (midnight) < 12:01 am, and 12:00 pm (noon) < 12:01 pm.
And probably last question: what is epoch? and why this Epoch since January 1, 1970 00:00:00.000 GMT.
Actually it is Unix time, or POSIX time, is a system for describing points in time: it is the number of seconds after 00:00:00 UTC, January 1, 1970.
Wait, one question more!
"If we use the time which is counted since Epoch, how can I know which years had leap seconds and which not?"
Answer: To make life easier leap seconds are not counted. Java Date class takes actual time from OS and most of modern computers can not use leap seconds, their's internal clocks are not so precised. That's why periodical time synchronization is required.
All Java code examples with Date class. (Calendar class examples follow below...)
All Java code Examples with Calendar class
Printer Friendly Page
Send to a Friend
Search here again if you need more info!