President Donald Trump thinks it’s a good idea if daylight saving time becomes permanent.

A federal law specifies that daylight time applies from 2 a.m. on the second Sunday of March until 2 a.m. on the first Sunday of November in areas that do not specifically exempt themselves. More than two dozen states are considering measures to avoid the twice-yearly clock change.

Sen. Marco Rubio and Rep. Vern Buchanan, both of Florida, introduced measures last week to make daylight saving time permanent nationwide. While federal law allows states to opt into standard time permanently — which Hawaii and Arizona have done — the reverse is prohibited and requires congressional action.

Trump tweeted Monday that making daylight saving time permanent is “O.K. with me!”

The law was first established during World War I as “a way of conserving fuel needed for war industries and of extending the working day,” according to the Library of Congress. But it was only temporary – the law was repealed as soon as the war was over.

But the issue of daylight saving emerged again during World War II. On Jan. 20, 1942, Congress re-established daylight saving time.

More than 20 years later, in 1966, former President Lyndon B. Johnson signed into law the Uniform Time Act, declaring daylight saving time a policy of the U.S. and establishing uniform start and end times within standard time zones. The policy is regulated by the Department of Transportation.

But not all states participate. Hawaii, most of Arizona and several U.S. territories—including American Samoa, Guam, Puerto Rico and the Virgin Islands—do not observe daylight saving time.