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Note the spelling. Java programmers rarely spell it correctly. This essay looks a little different from my usual
style. That is because was published in 2001 Volume 6 issue 6 of Java Developer’s Journal.
For newbies, dealing with dates and times are the probably the most confusing aspect of Java. There are three
reasons for this:
The date and time classes are the most poorly designed of all the Sun class libraries.
The standard class libraries force you to deal with time zone and time of day, even when they are
irrelevant to your problem.
The vocabulary used in the various date classes is inconsistent.
First, a quick review of what you probably were taught about the calendar in elementary school.
The earth rotates every 24 hours. The sun is highest in the sky at solar noon,
once each rotation, for any given spot on the earth. Railway schedules where hard to coordinate when towns even a
few miles apart celebrated noon at a slightly different instant in time. So, Sir Sandford Fleming invented
24 time zones, within each, time was uniform. Astoundingly, he managed to sell the
idea to the entire planet. The boundaries are essentially meridians every 360
÷ 24 = 15 degrees (1 hour). There are jogs in them to accommodate political realities.
The earth rotates from west to east. This means the British see noon, 5 hours
before New Yorkers who see it in turn 3 hours before the inhabitants of Seattle.
London is time zone UTC:0, New Yorkers EST/UTC-5, Seattle PST/UTC-8. In other words, to get Seattle time, you
take UTC (Coordinated Universal Time/Temps Universel Coordonné) (similar to GMT (Greenwich Mean Time)
) and subtract 8 hours.
Anyone who has flown over the Pacific ocean to Asia has crossed the International date line. When you cross it
going west to east, the day of the week jumps ahead a day. The planet does not simultaneously flip from Monday to
Tuesday. The people living just east of the International Date Line flip first, then an hour later the people
living 15 degrees east of that flip, and finally 23
hours later the people living just west of the International Date Line flip. So Tuesday washes over the earth
like a wave.
The entire planet is never entirely on Tuesday.
During wars, politicians wanted people to get up earlier to conserve electricity in the evening. They came up
with the bizarre idea of adjusting the clocks to trick people into compliance. This was originally called
war time, then later DST (Daylight Saving Time). It created chaos in many
ways. Part of the problem is the days for changing clocks forward and back and whether DST
is used at all are completely non-standard. Java has to maintain giant tables to track what each city on the
planet does with DST, and the history of what they have done in past.
The most important fact to drill into your brain is it makes no sense to ask what day of the week a given
instant in time (i.e. Java long timestamp) is. You must ask what day of the week it
was at that instant in a particular time zone. It also depends the particular location within that time zone was
at the time. Java uses micro-time zones, usually named after cities, each of which has a the same
rules and history.
The moon rotates around the earth once every 27 days and 8 hours, one lunar month (moonth), The Gregorian solar calendar we use today uses a
month of 28 to 31 days.
The earth rotates around the sun once every 365.242190 days, or one solar year.
We use a system of leap years with years of either 365 or 366 days so pretend a year is an even number of
Human biology is synchronised to the solar day, the lunar month and the solar year. The weirdness of our civil
calendars come from trying to pretend that lunar months and solar years are actually even multiple of days.
A highly accurate clock that monitors the frequency of light emitted by excited atoms or even a single
atom, originally cesium atoms, or clock that is accurately synched to such a clock.
day of month
The day of the month 1..31. Sometimes called the date.
day of week
Day of the week for a given date, e.g. Sunday = 1,… Saturday = 7.
offset. The number of milliseconds correction to account for daylight saving time, 0 if daylight saving time is
not in effect for the timestamp specified. If a one-hour daylight saving is in effect, the offset will be
3600000. You add the DST offset and the zone offset to UTC
to get local time.
A historic time standard derived by observing the sun at the prime meridian at the Royal Observatory,
Greenwich, England. It has been supplanted by the more stable SI (Systèm Internationale (metric))
standard based on an atomic clock.
Pope Gregory’s calendar we use today that has leap year corrections every 4, 100 and 400 years.
ISO day of week
Day of the week 1 to 7 for this date according to the ISO-8601 (International Standards Organisation # 8601 (date/time)).
Monday = 1… Sunday = 7.
ISO week number
Week number 01 to 53 of the year this date falls in, according to the rules of
ISO-8601. Week 1 of any year is the week that contains 4 January, or equivalently week 1
of any year is the week that contains the first Thursday in January. A week that lies partly in one year and
partly in another is assigned a number in the year in which most of its days lie. Most years have 52 weeks, but
years that start on a Thursday and leap years that start on a Wednesday have 53 weeks. January 1 may well be in
week 53 of the previous year! ISO specifies a lead 0 to create a 2-digit year. Java does not do that unless you
code the display specially.
an extra day added to the year to keep the calendar in sync with the revolution of the earth around the
month of year
January to December. Note that in GregorianCalendar, January is month 0. In contrast, in DateFormat,
January is month 1.
How many milliseconds difference local time is from UTC.
If you live is North America
this will be a negative number. It is the sum of the zone offset and the DST
offset. You add the DST
offset and the zone offset to UTC to get local time.
A TimeZone is a Java class for a region of the earth that keeps the same time.
If they keep different time in summer and winter, everyone is the region flips together. The TimeZone class describes the offset from UTC
in summer and winter time and when the flips occur of a given time zone.
There are many possible definitions. The default GregorianCalendar definition
depends on whether you consider Sunday or Monday as the first day of the week setFirstDayOfWeek, (the default is locale specific) and how many days you insist must be
present in the first week of the year setMinimalDaysInFirstWeek, (default 1). The
first week of the year is week 1. January 1 may sometimes be considered week 53 of the previous year.
milliseconds difference that local time is from UTC if you ignore any daylight saving time correction. West of
the prime meridian that runs through Greenwich England, i.e. in west Africa and North and South America, this
will be a negative number. Also Oceania (aka Oceania) in the Pacific Ocean east of the International Date line
will also have a negative offset. You add the DST
offset and the zone offset to UTC to get local time.
You need to use quite a few different classes to solve even a simple problem involving dates.
Used to convert a date to or from a String. Contains an associated TimeZone.
Used to convert a date to or from a String when you want precise control over the format. Contains an
Abstract class that is the mother of all Calendars such as GregorianCalendar. It owns the dozens of
magic date constants such as Calendar. JANUARY =
0; Calendar. SUNDAY = 1; and Calendar. YEAR = 1.
Sun’s first attempt at a Date class. I refer to it as the lemon of Java. It is now almost
completely deprecated. It now just basically just a wrapper around a UTC
date/timestamp long milliseconds since 1970. Unfortunately, it is still not
Used to do date calculations. Each GregorianCalendar contains a UTC
timestamp, and a TimeZone.
Contains the name of a time zone and how many hours difference from UTC
that time zone is. It also contains the rules for when daylight savings begins and ends. TimeZones are
named after cities. They are not the usual names.
Here in how to display today’s date.
Displaying A Date
This program will convert a date to a String for display, using the default date format. That format depends
on what the user has configured as his preferred date format in the OS (Operating System).
You can also use Formatter and printf.
ISO-8601 Date Format Rant
Java goes to great lengths to localise date formats, to display dates in the form preferred by each locale.
I think this a bad idea. Consider the following date: 11/10/09. What does it mean?
the 11th of October 1909, the 11th of October 2009, the
10th of November 1909, the 10th of November 2009, the
9th of October 2011? Everyone will interpret it differently if they see it embedded
in a globally accessible web page, especially the Wikipedia, book or magazine. And that is just the tip of the
Tower of Babel. We are a global village. We must use a commonly understood date format, and that is the
format yyyy-mm-dd. Thus 2011-11-02 means the
2nd of November, 2011 to everyone, unambiguously. If everyone used
ISO-8601 format it is would be much easier for search engines to find things by date and
to extract data factoids, such as birth and death dates.
In a similar way, the hodge podge of time zones and insane DST
rules means people are always getting confused by published dates and times. We need to standardise on a common
format to broadcast timestamps of events e.g. a coronation, namely in ISO-8601
standard format YYYY-MM-DDThh:mmZ so that 2011-11-02T20:46Z unambiguously means the 2nd of November 2011 at 8:46 PM UTC. Software in browsers could optionally convert such timestamps embedded
in the HTML (Hypertext Markup Language) back and forth to local time, much the
way Google Translate works, but without necessitating any server involvement.
DateFormat: Displaying A Date With Precise Control
If you want precise control of how your date looks, you can use a mask like this:
DateFormat Formatting Characters
DateFormat Formatting Characters
era: AD or BC
year, 2 digit
year, 4 digit
Month in year, 01-12
Month in year, Jan-Dec. Watch out! If your default locale is Locale.FRANCE this will come out in French
with accented letters.
Month in year, January-December. Watch out! If your default locale is Locale.FRANCE this will come out
in French with accented letters.
ISO-8601 week in year, 01 to 53
Week in month, first week is 1
Day in year. Jan-01 is day 1
Day in month, 1 to 31
ISO-8601 day of week, a number. Monday = 1… Sunday = 7
Day in week abbreviation, e.g. Tue. Watch out! If your default locale is Locale.FRANCE this will come
out in French with accented letters.
Day in week, e.g. Tuesday. Watch out! If your default locale is Locale.FRANCE this will come out in
French with accented letters.
Hour in day (0-23), 24-hour time
Hour in day (1-24), 24-hour time
Hour in day for AM/PM (0-11)
Hour in day for AM/PM (1-12)
Minute in hour, 0-59
Second in minute, 0-59
Time zone abbreviation, e.g. PST (Pacific Standard Time)
full Time zone, e.g. Pacific Daylight Time
Time zone offset e.g. -0700
Oracle’s Javadoc on Complete List of SimpleDateFormat Mask Characters : available:
Here is how to display today’s date in Zulu ISO-8601
Here is how to display local time, to the millisecond.
Parsing/Validating A Date
To convert a date from a String to internal format is quite a production. This
technique will also validate a date. Just catch the ParseException.
Be very careful with TimeZones. If you don’t specify one, your date will
be interpreted using the local default TimeZone. If the user has not configured it
correctly in his OS, you may get Pacific Standard time or GMT,
Elapsed Time in Years, Months and Days
Elapsed Time In Hours Between Two Timestamps
Have a look at this example program to calculate how many hours until the next presidential inauguration.
You specify the timestamp with GregorianCalendar.set
in terms of Eastern Standard time, not UTC. Internally the timestamp is stored as
You don’t create the TimeZone object with new.
The getTime().getTime() is not an error. The first getTime retrieves a Date object from the GregorianCalendar, and the second retrieves a timestamp from the Date
Elapsed time in days is not simply hours / 24. It is much more complicated than that.
How Long Until Christmas, Daddy?
This sounds like a simple problem. Programmers posted many different solutions to the
newsgroup before the gurus stopped finding holes in the logic. Part of the problem is that the question can have
many different legitimate answers.
Taking A Timestamp Apart
Building A Timestamp from the Pieces
// building a timestamp using default TimeZone.GregorianCalendar stamp = new GregorianCalendar();stamp.clear();stamp.set(year, month-1, day, hour, minute, second );long timestamp = stamp.getTimeInMillis();
java.util.GregorianCalendar has far fewer bugs and gotchas than the old java.util.Date class but it is still no picnic.
Had there been programmers when Daylight Saving Time was first proposed, they would have
vetoed it as insane and intractable. With daylight saving, there is a fundamental ambiguity. In the fall when
you set your clocks back one hour at 2 AM there are two different instants in time both called 1:30 AM
time. You can tell them apart only if you record whether you intended daylight saving or standard time with the
reading. Unfortunately, there is no way to tell GregorianCalendar which you
intended. You must resort to telling it the local time with the dummy UTC
TimeZone to avoid the ambiguity. Programmers usually close their eyes to this problem and just hope nobody does
anything during this hour.
Millennium bug. The bugs are still not out of the Calendar classes. Even in
JDK (Java Development Kit)
1.3 there is a 2001 bug. Consider the following
GregorianCalendar gc = new GregorianCalendar();gc.setLenient(false );/* Bug only manifests if lenient set false */gc.set(2001, 1, 1, 1, 0, 0 );int year = gc.get (Calendar.YEAR );/* throws exception */
The bug disappears at 7AM on 2001/01/01 for MST.
GregorianCalendar is controlled by a giant of pile of untyped
int magic constants. This technique totally destroys any hope of compile-time error checking. For example to
get the month you use GregorianCalendar. get(Calendar.MONTH));
GregorianCalendar has the raw GregorianCalendar.get(Calendar.ZONE_OFFSET) and the daylight savings GregorianCalendar. get( Calendar.
DST_OFFSET), but no way to get the actual time zone offset being used. You must
get these two separately and add them together.
GregorianCalendar.set( year, month, day, hour, minute) does not set the seconds to
DateFormat and GregorianCalendar do
not mesh properly. You must specify the Calendar twice, once indirectly as a
If the user has not configured his time zone correctly it will default quietly to either
In GregorianCalendar, 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. Yet DateFormat. parse behaves in the traditional way with January=1.
( Locale. ITALY) will not have any effect on
SimpleDateFormat objects already created. You need to set the default Locale before creating your SimpleDateFormat objects or use the
SimpleDateFormat constructor that takes a Locale.
launch = Date.parse("Fri, 20 Nov 1998 00:20:00
CST"); will, in Opera, for example, be interpreted ignoring the CST, taking the default time zone instead.
Designing a Calendar
Designing calendars is an inherently messy problem.
You are try to measure years, days, lunar cycles, each of which only approximates a constant unit of
measure. There is no way to do it with a single scheme.
You also need a measure that is not connect to astronomy, something as invariant as possible for doing
things like measuring the wavelength of light emitted by various excited atoms.
Tradition clings to calendars long after they have proved unworkable.
Religious people and other zealots think they have the right to redefine calendars.
Roman politicians meddled, stealing days from February to make July and August longer.
Unix has defined a simple system of measure in milliseconds plus or minus 1970-01-01T00:00. It is effectively a day-counting system. It needs no months, weeks, years or
other complicated structure. Perhaps over time it will supplant the other calendars. You can use simple
addition and subtraction to do date calculation.
Here is a list of available time zones:
Standard : (DaylightSaving) : TimeZone
Here is a list of available TimeZones:
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Add column 1 in hours to UTC to get local standard time.
Add column 2 in hours to UTC to get local daylight saving time.
Use UTC when you want no time zone at all.
Use Asia/Riyadh for Arabia
Standard Time. Asia/Riyadh87, Asia/Riyadh88
and Asia/Riyadh89 are 3 hours and 7 minutes east
of UTC. This is the offset used in 1987 to 1989.
Actually it was 3 hours 7 minutes and 4 seconds to approximate solar time. Prior
to 1950 they used 3:06:52. In the period 1951—
1986 and 1990 onward they
used a simple 3 hour difference. In Islamic tradition, the day starts at sunset.
Note how much many aliases there are for the same time zone. I speculate there
are two reasons for this:
To anticipate some geographical region adopting a quirky daylight saving rule in
future. There would be no need adjust to a new split time zone to adopt the new
rule. Only the tables built into Java would need to be adjusted.
It makes it easier for people to find their own time zone. They need find only a
nearby city without having to consult a map to find a distant city on the same
longitude. Often they can find their own city directly.