Puzzler, for the stamp you showed, that is the correct date/time; you simply add the emperor year to 1988 to get the correct year.
For stamps issued before 1988, the year of the emperor is added to 1925.
There is a small complication to this, though. The Showa Emperor (Hirohito) died during
1989, so regnal year dates for 1989 up until his death, show Showa 64. After his death and the ascension of Emperor Heisei, they show Heisei 1. I don't think it appears on Japanese CDSs, but another way of indicating 'First year of Heisei' or 1989, is #24179;#25104;#20803;#24180;.
(Note that to convert to English dates, you count from 1925 for Showa and 1988 for Heisei, although they ascended in 1926 and 1989 respectively, because the Japanese count inclusively.)