The transitions between the different major stylistic periods in the history of music rather than happening overnight were normally rather gradual transformations, mirroring other similar transformations in society and the arts. The last composers of some of the principal musical epochs were sometimes also the ground-breaking first composers of the new epochs. For example, Schubert and Beethoven, who began their “careers” writing in a very “Classical” style could also, at the end of their lives, be considered as the first “Romantics”, giving us two very good examples of composers who can be classified in more than one stylistic period. Other composers such as Saint Saens, Brahms, Rachmaninoff and Elgar stayed loyal to their stylistic traditions during their entire life even though the musical world around them was evolving rapidly.
This is why it is so difficult to put dates on the changes from one stylistic period to another. We can see this especially clearly in the transition from “Romantic” to “Modern” music. While Elgar, Saint Saens, Rachmaninoff and Richard Strauss continued composing music that could only be called “Romantic” well into the 20th century, other musicians were already writing in the totally different “Modern” styles of ragtime, jazz, impressionist, atonal and dodecaphonic (serial) music.
Here are links to the cellofun editions of music from the principal stylistic periods: