According to Wikipedia, Kenya was made a Crown Colony of Great Britain in 1920 and remained in this status until its independence on December 12, 1963. Before 1920, Kenya was within the British East Africa Protectorate. It states that the republic was established a year after the 1963 independence and I infer from this that the Republic of Kenya formally dates from 1964.
Unless I overlooked it, I did not see the exact answer to Red Steel's question on what Kenya was formally or informally known as during the ca. 1 year between independence and the Republic of Kenya being formalized. http://en.wikipedia.org/wiki/History_of_Kenya
http://en.wikipedia.org/wiki/Kenya
Interesting that the Kenya wiki page shows very recent editing:
‘’This page was last modified on 2 August 2009 at 12:08’’
Some called it the “Dominion of Kenya”, but I suspect that that is a name that would not have found favor in kenya itslef.
So far online using Google, about the only sites that refer to Kenya being called “Dominion of Kenya” between Dec 12, 1963 and Dec. 12, 1964 are flag, coin, and Wikipedia sites. I’ve tried using my Lexis-Nexis connection to find the earliest dates of both “Dominion of Kenya” and “Republic of Kenya” but the site keeps crashing on me. I’ll try later at the lab.