What date did Stalin become leader of the USSR?

Vladimir Ilyich lenin (Ulyanov) was effectively removed from power in 1922 by the first of a series of strokes. Since he had failed to name a successor, the USSR was run by a "Committee of Three" including Grigory Zinovyev, Lev Kamenev and Joseph Stalin. Stalin held effective power at this time because he was already Party General Secretary. There is a little doubt as to what exactly happened from here on because many records were "corrected" to reflect the preferred political views of the time, but suffice to say that by the end of 1929, only Stalin was left of the "Committee of Three" and he had managed to get rid of the remainder of his potential competition from the party. Both Grigory Zinovyev and Lev Kamenev were executed in 1936. The rest is history and Stalin allegedly died of a stroke, possibly on the 5th of March 1953, thereby resigning his leadership of the USSR. Exact dates, even if they do exist, are not reliable.