Since I wrote my first Traditional Regency Romance I have accumulated a lot of non-fiction books about the era. Today I piled them up around me and began to answer the question when, i.e. when were the main characters born, when did the hero go to school, when did he go to university, when did the heroine's husband die, when was her posthumous son born, etc.
Sunday's Child ended in 1815 when news of Napoleon's escape from Elba arrived. So it's a question of working backward to fill in the correct dates on my character profiles.