Date and time in Java. During the course of my career I have met (too) many developers who are not familiar enough. I am pointing all developers with knowledge deficiency. This absence of knowledge can manifest in different statements: You want a time zone in the date format? No problem, I'll just always append +0. We don't need time zone information because we don't have overseas customers. We don't need time zone information because Switzerland doesn't stretch across time zones. No, there can't be a time zone issue: Webserver and DB are in the same room. Why is there a 'Z' at the end of this XML datetime string? As with so many things everything said here is only true to some. The precision here ends with leap seconds. I am not taking these into account as in. The Date class, found in the java.util package, encapsulates a long value representing a specific moment in time. One useful constructor is Date(), which creates a Date object representing the time the object was created. The getTime() method returns the long value of a Date object. Java Date Conversion Program PdfIT systems they are not relevant and our clocks are not as accurate. If you work in astronomy. What is time? Time is a physical property. For Physicists it is a dimension tightly coupled with space. Time as a physical property (or dimension) passes uniformly. And to any. precision that is relevant to us. Even if it did not we were probably unable to tell. We simply can't define how fast time passes - we live. Just like a piece of driftwood in a river is unable to change its speed relative. Date's getTime() method returns a long value whose number equals the number of milliseconds before or after January 1, 1970. The above calculation is shown in the following Java program: import java.util.*; public class Elapsed1 Java Date conversion 0 Answer(s) 8 years ago Posted in : Date-Calendar Ads View Answers Post Answer. Display Date Plz send me sample java program that display day, date. This example * use SimpleDateFormat for String to Date conversion, you can also. Because. to measure speed you compare the difference of what you measure to a difference of time. So we can safely assume that physical. These are notions defined by. Measuring time. We don't live inside a particle accelerator. We know day and night. We can. see time passing with every day, month, year. These are marked by periodic events: sunrise, full moon.
That's the way we have been measuring time forever: we use natural periodic events. Here the. periodic events are the waves of (a very precise) electromagnetic radiation. Note. that, lacking any other option, our definition of the second is completely arbitrary. We live on Earth, which spins around its axis. This is an accelerated motion by. General Relativity, which means that we measure (slightly) shorter. But as stated above our definition of the second. So as long as you don't work for NASA and as long as SETI hasn't found. Measuring duration. So we employ the definition of the duration of a second to measure the passing of time. We can. notably use seconds (and its fractions) to measure durations: how long it takes us to go shopping. A duration is the answer to the question . You. measure durations with a stop watch. Java Date Conversion ProgramsSpecifically a duration does not answer the question. We can do this simply by laying out a time scale much like. A ruler has a zero mark and a positive (and maybe negative) part on its scale. We can do. the same in time. We define some arbitrary point in time as 0. From that we can make a reference. It gives every second on that scale a name. It never goes backwards. Some. minutes however may not have exactly 6. Some may have 5. 9 (that has not happened so far). They are called leap seconds and are used. Computer clocks. Computers contain clocks (also referred to as timers). Several of them usually. With different precisions and capabilities. This method uses interrupts to get a timeout notification. The precision of this timing may vary widely. Their speed varies. They need to be adjusted all the time to avoid going either too fast or too slow. With some millisecond precision. Servers whose time is not in sync with the global time cause. The UNIX NTP daemon can keep time in sync with. This minimal /etc/ntp. UDP port 1. 23 in your firewall open). Local time. When you look at your watch, most of you do not read UTC. Also known as wall clock time. More on this later. Ambiguous Day. This may seem obvious, but it is not. A day is defined as 8. This is the time the. Exactly it is the time until the sun appears in the same place. This is different from the time it takes. As you will see in the. Daylight Saving Time a day in your local time is not always 2. In many countries. Applications should never assume that a. The method. java. Timer. schedule(Timer. Task task, Date first. Time, long period) is unsuitable. If you pass 8. 64. DST change the timer appears to be firing one hour off. While the timer sticks exactly to its 2. Representing points in time by UNIX time. In IT systems we sometimes need to refer to precise points in time. Examples are: When was a file last modified. Start of an appointment. When an email was sent. These points in time have the characteristics of an event. If two events have the same time. It doesn't matter if one occurs in. Shanghai and the other in London: they still occur in the very same moment. Independent of any. We usually use UNIX time to represent points in time. UNIX time stamps are the number of seconds that have elapsed since midnight 1. UTC. Never mind, we'll get to that in a minute. It represents. a point in time by a millisecond counter in a 6. While 3. 2 bit representation. UNIX will overflow in 2. They are from a time when the Sun people were confused about time themselves. Be careful of int overflows! But it is far from usable. That's why we use calendars. Not only in the western world today. Gregorian Calendar. It defines twelve months a year of different length and. February. It is a. July in. Switzerland not because of an aberrant calendar but much more likely because of a change in climate. Calendar and time zone. Calendar define dates. A calendar is. tightly coupled to the notion of a . Sunrise and sunset depend on where on earth you are, don't they. This is especially visible on New Year's eve when around the globe people launch fireworks. The toasting and launching of fireworks happens roughly 2. It is also apparent that at any given moment there is not just one date around. While people in Europe are already drunk and celebrating January 1st, people in. New York are just starting to cook dinner on December 3. It can refer to any point in time within 2. All of the following expressions are useless without a time zone! Swiss local time (see Daylight Saving Time paragraph)Monday. Ambiguous Day paragraph)Time zones. You have certainly heard of time zones. But do you really know exactly what they are? Comparable with. the unit of measure for . A stick of 1 meter length is 1. The point in time doesn't change whether I express it in one time zone or another. The mother of all time zones is. Greenwich Mean Time (GMT). Offsets of all other time zones are specified in. GMT. Zones to the east have positive offsets, zones to the west have negative ones. Other units are expressed as multiples of the meter: 1 millimeter = 1/1. Converting between time zones is easy. GMT = 1. 8: 0. 0+0. The suffix is the offset from GMT that has been added to the GMT time. The Java java. util. Time. Zone can represent two things depending on how it is used. A time zone. A time zone database of a location. Time zones are not assigned to locations by astronomers but by politicians. That's why that. time is called legal time. These assignments. That's why Time. Zone keeps a database. So their time zone is not always GMT! Most of the time applications use a location database like. Time. Zone. get. Time. Zone(. This frees the application from most DST issues. Of course an application can also use a simple exact time zone if required. Time. Zone. get. Time. Zone(. If you don't specify the time zone, Java will. In a server environment that is hardly the one that's right. Localizing an application doesn't just mean to support different Locales, but. Time. Zones. So classes in your codebase that are Locale dependent, are. Time. Zone dependent. It should be noted that Time. Zone. get. Time. Zone() does not throw an exception. Instead it returns GMT. Otherwise your application code will behave unexpectedly. Representing a point in time in different calendars and time. The only time a representation. When a human needs to read a. In Java we use the java. Date. Format and it's most used implementation. Simple. Date. Format. Date instances to and from user readable time representation. Important here is. Otherwise the system default time zone will be used which. Desktop applications. Also note that Simple. Date. Format instances are. You should thus avoid static instances. It is ambiguous without AM/PM. Furthermore the Calendar class doesn't. Calendar. HOUR, 2. It allows to convert to and from Date with the set. Time and. get. Time methods. It also allows modification of individual fields. This is the class. Also here, set the right time zone or. DST change dates are handled. That's necessary as these days don't. But for most of the people the 2. Some people (especially in central Europe) are. Imagine you hand out plane tickets with a boarding. You can be sure that half the passengers will show up half past. As a rule of thumb: There is no zero hour on the 1. AM. So. 1. 2: 0. 0 AM is equal to 0: 0. PM is equal to 1. I strongly suggest you use the 2. AM/PM notion. Or at. On printed documents, such as tickets, you should never. In Java this means to avoid Calendar. HOUR, Calendar. AM. Instead use Calendar. HOUR. This is. semantically impossible as a Date is always in UTC. Only a Calendar or a textual. In practice time zone. In these cases a java. Date. Format should be. It has no effect and demonstrates a lack of understanding. Date and Calendar. This method does nothing. They refer to more than a certain second. Some of these designate the duration of a single day. Others several days. Representing a period. Periods of time can have very different semantics. They may require different representations: Begin and end of a Java method call: Two UNIX time stamps The meeting is from 8h. UNIX time, duration in minutes. During my vacation: Two Calendar objects with time zone. The date a piece of hardware was repaired: Calendar object with time zone. My mother's birthday: Calendar object with time zone. When I was young: fuzzy correlation between . This way it is possible to represent consecutive intervals without gaps. For. representation to a user it may be necessary to specially handle boundary cases. Consider the following situation: A calendaring web application stores. DB. So the next occurrence is .
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. Archives
December 2016
Categories |