In I Saw the Sun Rise (2018), it was established that Magnum had been working as a Private Investigator for several years. In the present storyline several people referred to him as only being in business for three years which would have been the beginning of the series.
Magnum prefers to be called a "Private Investigator" and hates when someone refers to him as a "P.I.". In the flashback, he referred to himself as a "P.I." twice.