What is the narrator point of view in the novel, Killers of a Certain Age?

The novel features a dual timeline. One series of events take place from 1978 through 1981. The other timeline takes place in modern times, but the author does not identify the exact year. The events that take place around 1980 are presented in third person but is limited to Billie's perspective with a few minor exceptions. There are obviously events that are taking place at this time outside her perspective, but the reader does not know any details of those events. The later timeline is also from Billie's perspective, but this is written in first person. There does not seem to be a real reason for that other than the possibility that Billie has aged and matured in the meantime. This could be an indication that she considers herself a different person looking back on her past.