• java.util.Date : for handling date/timestamps presuming GMT. It is the lemon of Java. It is mostly replaced by GregorianCalendar. It is used mainly as if it were a Long wrapper for a date/timestamp.
• java.util.GregorianCalendar : a date/timestamp and timezone. The timezone is usually ignored.
• java.util.SimpleDateFormat : for converting dates to and from human-comprehensible strings.
• java.util.TimeZone : represents a timezone. Watch the capital Z. Watch out, Java does not name the timezones in the way Americans are familiar with.

// get time in milliseconds since 1970 Jan 1
long timestamp = System.currentTimeMillis();

There are a ton of surprises and bugs awaiting the unwary users of these classes.

java.util.Date Gotchas

The key to understanding Date is that it is not a date class, but a timestamp class. Inside each date is stored the number of milliseconds since 1970-01-01 UTC. It does not record the timezone or the timestamp in local time. You choose the timezone when you display the date with code like this:

In contrast GregorianCalendar objects do contain a TimeZone reference along with the date and time. Like Dates, they are initialised with the current date and time. Unfortunately, if they are not sure about the user’s timezone, the Sun classes quietly revert to GMT, and in early versions of GregorianCalendar, seem to revert to PST if you so much as breathe on them. DateFormats also have TimeZone that is similarly erratic. For code to work, your clients must have configured the TimeZone correctly in the OS. You can’t help them configure it.

• Inconsistent capitalization. java.util.TimeZone, but Date.getTimezoneOffset.
• Inconsistent naming. We have System.currentTimeMillis but Calendar.getTime InMillis. Note also that new Date objects are initialised to the current date and time, not to a null value. This gives you two ways to get the current time.
• Documentation is never clear on when you are using local and when UTC.
• Months are numbered starting at January=0, rather than 1 as everyone else on the planet does. Yet days start at 1 as do days of the week with Sunday=1, Monday=2,… Saturday=7. GregorianCalendar uses the same idiotic convention. Be thankful for small mercies. DateFormat.parse behaves in the traditional way with 1=January, though reputededly its isLenient mechanism does not work.
• getFirstDayOfWeek return Sunday=1 in the USA and Monday=2 in France.
• Timezone.getOffset wants a Sunday=1-based day of week, which it ignores other than sometimes to check validity. The bug only shows up on Sundays with an InvalidArgumentException in some JVMs if you pass it a Sunday=0-based dayOfWeek. Your bug can thus easily leak through into production.
• Dates prior to 1970 are not handled in JDK 1.0 but are handled in JDK 1.1 and later.
• Monday is day 2, (half expects Monday to be day 0, the other half expects it to be day 1, only the folks at Sun expect 2.). This is not properly documented.
• For Date, year 0=1900, year 100=2000. This has been fixed in Calendar. Be careful if you are using both Date and Calendar in the same program.
• There is no reserved value for a null date.
• Dates are stored internally with a date and time in GMT, and automatically converted to local time. If you are not careful a date stored as Saturday can come back a day earlier or later.
• In JDKs before 1.1.7, DateFormat uses PST (Pacific Standard Time) as the default not GMT or local time as you might expect. To make it use local time you must do:

  DateFormat.setTimeZone( TimeZone.getDefault() );

  To get UTC, without any daylight saving correction, you can use:

  DateFormat.setTimeZone( TimeZone.getTimeZone("UTC") );

  You must configure your TimeZone property in your OS (for example, in the Windows Control Panel) for TimeZone.getDefault to work. The default timezone will be automatically configured in Java once you configure the correct timezone in your OS. However, selecting the right timezone is more than just selecting how many hours off GMT you are. You have to select the correct set of rules for when daylight savings changes. Java does everything internally in GMT and converts for display, so it is not enough just to manually reset your PC clock in the spring and fall. The configuring process can fail for exotic parts of the earth because the timezone tables either in the OS or Java are imperfect. Unfortunately, if you actually live in the GMT timezone, Java will think you want GMT standard time without any daylight savings changes. The best solution to this I know so far when your timezone is not properly supported is to roll your own TimeZone object with the proper daylight savings switchover dates.
  Alternatively, for applications, you can override the timezone on the java.exe command line with something like this:

  java.exe -Duser.timezone=Europe/London HelloWorld

  Make sure the -D goes before the classname, or it will be interpreted as an arg to your HelloWorld class. You may have to do this if the JDK does not properly recognise the name of the timezone that your OS calls it. The names for timezones used in Java comes from a list maintained at NIH by Arthur David Olson. Unfortuntately, the common names such as EST can mean
  Eastern Standard Time: GMT+10 (in Australia)
  Eastern Standard Time: GMT-5 (in the US)
  so they are not suitable for international use. Pacific Time is called America/Los_Angeles instead of America/PST. You have to get out an atlas to find out if you are north of Los Angeles or South of Alaska. The idea is America/Los_Angeles means PST/PDT, as appropriate. PDT would imply a daylight saving correction even in winter. It would have made more sense to call it America Pacific Time or America PST/PDT. America/Los_Angeles only has meaning for Californians. When testing timezone-sensitive code, keep in mind that the switchover between daylight and standard time does not happen at midnight.
  TimeZone: a list of available timezones, and how your machine in currently configured
• The people who wrote Date and Calendar need to be sent to a class in English composition. The key to clarity is a consistent vocabulary. They seem to think day, date, time and timestamp are synonymous. They muddle them hopelessly. The Date class is actually a timestamp, not a date. The method names are misleading. Date.getDay gets you the day of the week. GregorianCalendar.get(Calendar.DATE) or Date.getDate gets you the day of the month. Date.getTime gets you a date/time stamp milliseconds since 1970, in Date, but gets you a Date object in GregorianCalendar. java.sql.Date and java.sql.Timestamp are just thin wrappers around java.util.Date. As such, you can interconvert using the getTime and setTime methods with a long milliseconds since 1970 Jan 1 intermediary, or simply cast to Date. In JDK 1.4+ the Calendar.getTimeInMillis() has been made public, so you can do it in one step. Date/Calendar needs to be rewritten to use consistent vocabulary. Roman emperors and popes started the calendar out in a rather confusing manner. Oracle uses Date to mean a timestamp. C uses the January=0 convention. Sun authors had a failure of nerve to break with Byzantine traditions of their predecessors.
• Dates and Calendars are bulky. They are not a suitable way to carry around date information in RAM. Happily, Date boils down to a Long when serialised for external storage. There needs to be a minimalist core TimeStamp class, that has no internal baggage, for storing raw TimeStamps. In the meantime, I recommend you store them as longs or Longs.
• Within the Date class, even though the dates are stored in GMT, they come out field by field in local time, e.g. when using getDay and getMonth. there is a way to find out which local time it is using with getTimezoneOffset, but there is no way to set the timezone you want to use. You can set the default local timezone when formatting, but it does not apply to your calculations based on getMonth etc. The calculations you are doing may have nothing to do with local time. To add insult to injury, if the client did not properly configure the timezone, the calculations could have been quietly done relative to PST (early JDKs) or GMT, in more recent JDKs. BigDate sidesteps the entire issue by working with pure dates, no times no timezones.
• The list of possible timezones is incomplete and ambiguously defined. For example, "BST" as a timezone refers to Bangkok Standard Time, not British Summer Time. In Solaris, when you want to express your date in the MET timezone (continental european timezone), you get GMT + 3h30 : Teheran time.
• DateFormat.parse is broken in JDK 1.2.1. It is fixed in 1.2.2.
• Oracle SQL databases expect dates stored to be presented in UTC, yet when you retrieve them they automatically convert to local time. This is not what you want. A servlet’s local time has no relation to the final client’s local time. Store dates as longs or ints to avoid Oracle’s foolish meddling.

Learning More