According to the Corpus of Historical American English, 1911 was right in the middle of the transition from to-day to today, with to-day having 95% market share in 1890, 80% in 1900, 70% in 1910, 30% in 1920, 20% in 1930, and 5% in 1940: http://corpus.byu.edu/coha/?c=coha&q=12896286
(Can't use Google n-grams on this one, because its punctuation-stripping makes it impossible to distinguish to-day from to day.)
