Since the Meiji Restoration, the Japanese calendar has been effectively identical to the Western one. The Japanese traditional calendar would also make an interesting node, but as it is entirely obsolete, I will limit myself to the modern system -- as you are about to find out, there is more than enough to the art of reading and writing Japanese dates as it is...

As in addresses, Japanese dates are sorted from largest unit to smallest, in the order year/month/date, with slash and period used pretty much randomly to separate the units. Thus, December 7, 2001 would be written 2001.12.7, or 2001年12月7日 in Japanese characters. The character representing the day of the week is often tagged on at the end in parentheses, as in 2001年12月7日(金) for "Friday, the 7th of December 2001".

Now let's take a detailed look at each element:

Year

The one major holdover from the traditional system is the system of using eras, corresponding to the Emperor's reign, for years. While the Japanese are familiar with the Gregorian calendar and use Western years (西暦, seireki) in all international and some official correspondance, the Imperial system remains ubiquitous in Japan itself.

The way the system works is that upon ascending to the throne, the new Emperor's era is given a name. The current era, which started in 1989, is Heisei (平成). The first year of an era is called gannen (元年), which means "initial year", and after that the years simply count up. The Imperial year is synchronized to the Gregorian calendar, so subsequent years of a reign start on January 1st. Thus 2001 is Heisei 13-nen (平成13年), "the thirteenth year of Heisei", read Heisei jûsannen in Japanese. Note that Chinese numbers are always used, see Japanese numbers and counting for the gory details. It is also not uncommon to see these years as numbers alone, eg. an expiration date of "13/12/11" means December 11th, Heisei 13 -- and yes, this can be confusing sometimes.

One slightly confusing bit: the new era starts immediately on the new Emperor's ascension, so 1989 is known both as Showa 64 and Heisei 1. Technically, Showa 64 is the bit before the Emperor's death and Heisei 1 the bit after, but in practice the two are synonymous. After his death the Emperor is referred to by the name of his reign, so the current emperor is just tennô heika, "the Emperor", but Hirohito is known by the Japanese as Shôwa tennô, "the Showa Emperor".

The names of the four eras after the Meiji Restoration are in common use. They are:

The era names are effectively meaningless, which is why memorizing all 247 of them is not expected even for the Japanese; the pre-Meiji ones do not even necessarily match the emperor's name. In modern Japanese documents, any reference to dates in the pre-Meiji era will include both the imperial year and its Gregorian equivalent.

Finally, note that Imperial dates are used to refer to the future, eg. Eidan expects to complete Line 15 of the Tokyo subway by the nineteeth year of Heisei (2007). Dates more than a decade in the future do tend to be written in Gregorian years though.

* Cut it with the AYB jokes already and go read about what the Emperor of Great Justice did in Manchukuo instead, willya?

Month

Months are considerably easier: they consist of the Chinese number with the suffix -gatsu (月, "moon") tacked on. The full table is:

 1  January     ichigatsu
 2  February    nigatsu
 3  March       sangatsu
 4  April       shigatsu
 5  May         gogatsu
 6  June        rokugatsu
 7  July        shichigatsu
 8  August      hachigatsu
 9  September   kugatsu
10  October     jûgatsu
11  November    jûichigatsu
12  December    jûnigatsu
The only exception is September's kugatsu instead of the expected *kyûgatsu. Even April and July stay as shi and shichi, instead of being yon and nana as they are in almost everything else due to superstition.

Day

Writing days is easy, just tack the kanji for "sun" (日) onto the numerals. Reading them, on the other hand, is a real mess. The basic rule is that the first ten days are counted with Japanese readings, with the suffix being -ka, and the rest with Chinese readings, the suffix being -nichi, but there are more exceptions. The full list with exceptions in bold goes like this:

 1  tsuitachi
 2  futsuka       (futatsu)
 3  mikka         (mittsu)
 4  yokka         (yottsu)
 5  itsuka        (itsutsu)
 6  muika         (muttsu)
 7  nanoka        (nanatsu)
 8  ka         (yattsu)
 9  kokonoka      (kokonotsu)
10  tôka         (tô)
Let's take a breather here -- as you can see, the dates are derived from the Japanese numerals with -ka substituted for -tsu, but there are still plenty of bizarre changes, and this list pretty much has to be memorized. The rest is easier, with -nichi following the Chinese numeral systematically (eg. jûichinichi for day 11), except on the following days:
14  jûyokka
20  hatsuka
24  nijûyokka
...hatsuka being ancient Japanese for "20" and shi, "death" as well as "four", being avoided. And finally, January 1st is known as ganjitsu.

For the student of Japanese, this system is really a pain in the butt when e.g. arranging reservations over the phone. Practice and practice it well.

Days of the week

The idea of a seven-day week was also instituted during the Meiji Restoration, as previously the Japanese had been using a six-day week (which still lives on in fortunetelling). The days were taken from the traditional seven luminaries (sun, moon and five planets) with the suffix -youbi tacked on, in this order:

Monday     getsuyôbi   moon, Moon
Tuesday    kayôbi      fire, Mars
Wednesday  suiyôbi     water, Mercury
Thursday   mokuyôbi    wood, Jupiter
Friday     kinyôbi     gold, Venus
Saturday   doyôbi      ground, Earth
Sunday     nichiyôbi   sun, Sun
Interestingly enough, these correspond quite well with the planets assigned to the various Roman and Norse deities that form the basis of the English weekdays, and this was probably taken into account when assigning the order of the days. In China, the communists opted to drop such theist thoughts entirely and simply numbered their weekdays instead.

Putting it all together

A few examples:

Sunday, December 9th 2001

Heisei jûsannen jûnigatsu kokonoka (nichiyôbi)
Thursday, September 1st 1977
Showa gojûninen kugatsu tsuitachi (mokuyôbi)
And a bow to sekicho for comments.

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