The date of a Japanese print can often be ascertained from the censor seals on it - at least for prints which were sold publicly, and thus had to pass the censors. (Private, limited edition prints such as surimono, as well as outlaw prints such as shunga, were evidently condoned - or overlooked - if issued discreetly.)
From 1790 until 1876 (which roughly parallels the last half of the best of ukiyo-e, and the period from which prints are most commonly found today), when formal censorship ceased, all woodblock prints had to be examined by official censors, and marked with their seals.
The changing forms of these seals, as they government changed the censorship
process, and the seals applied as part of that, allows the dating of prints
to a varying degree of accuracy. The general form of the seals almost always
gives a rough idea of when the print was produced. During some periods, the
details of the censor seals allows dating a print to within a month.
To understand the ability to do such accurate dating, one needs to understand that the Japanese used (along with several other calendrical systems!) the Chinese zodiacal calendar, which is a twelve-year cycle, with each year having an associated animal, e.g. rat, ox, etc. (Anyone who has ever eaten in a Chinese restarant will no doubt recall this from the placemats!)
During large parts of the censor seal period, the seals
a zodiacal year indication, along with the numeral for the month. Although
this only technically narrows it down to a group of years, spaced twelve
years apart, other indications (e.g. the artist's working period, or the form
of his signature, or the form of the censor seals, which changed often)
usually allow one to ascertain the exact date.
Broadly speaking, the general form (i.e. number, contents, shape, etc) of the censor seals used in the period from 1790 to 1876 (during which woodblock prints were censored) falls into five main periods. Within each of these periods, the exact form of the seals (i.e. square, round, oval, etc) often changed; this level of detail is only useful for actually dating prints.
(Not covered here are certain obscure censor seals used for special forms, e.g. fan prints).
(One uncommon type from this period, one which is easy to confuse with later seals, was used during the period 1815 to 1832, and includes a zodiacal date in the top part of an oval seal, along with an aratame ("examined") character below.)
From 1847 to 1853, the Nanushi marked prints in pairs; an oval aratame seal is often present during the period from 1848 to 1851.
In the initial phase, a circular aratame seal was used along with a separate oval zodiacal date seal. In 1858, the aratame seal was dropped, and the oval date seal appears alone.
In 1859, the seal was changed so that both the aratame and zodiacal date appear in a single round seal. The arrangement of the contents of this single seal changed over time: in the initial arrangement, the year sign was as the top, and the month numeral at the bottom enclosed the aratame character; in the second arrangement (found most often from 1859 to 1863), the aratame character is at the bottom left, with the month on the top left, and the year on the right; in the third arrangement (found most often from 1864 to 1871), the aratame character is on the left, with the year sign on the top right, and the month on the bottom right.
James Self, Nobuko Hirose, "Japanese Art Signatures: A Handbook and Practical Guide", Tuttle, Rutland, 1987This invaluable reference work is now unfortunately out of print, and completely unavailable on the used book market, which is why the information is being made available online here.