Japanese covers and postcards quite commonly have the date based on the Japanese calendar system rather than that of the Julian calendar. The following table is a guide to correlating between the two systems.
| Japanese Period | Julian Calendar |
| Edo | 1603 to 1867 |
| Meiji | 1868 to 1912 |
| Taisho | 1912 to 1926 |
| Showa | 1926 to 1989 |
| Heisi | 1989 to today |
Thus, for example, if postmark has 23 year date but stamp wasn't issued till 1937 it is from the Showa Period.
So take 1926, add on 23 to give 1949, then minus one for the overlap to give Julian equivalent of 1948.
Since Taisho and Showa are both in 20th century, an easier formula is to add 11 to Taisho date and 25 to Showa
date to get Julian years AD.