When was the national flag adopted?

As with the national anthem, there were no laws designating the Rising Sun as the national flag until the National Flag and Anthem Law made it official in August 1999.
The current design - a red disc in the center of a white field - was originally adopted by the Tokugawa government in 1854 to be used on flags for Japan's ships.This was done in order to distinguish Japanese ships from foreign ships, and was not necessarily intended to stand as Japan's national flag.
During the Meiji period, in the year 1870, a proclamation issued by the Dajohkan (Grand Council of State) adopted this as the standard flag for ships, and over time this became accepted as the national flag.
As the Rising Sun has come to symbolize a Japan that once traveled an erroneous militarist path, there are now in the postwar period a number of people who, as with Kimigayo, reject the Rising Sun.