The McMillan family came to Utah between 1863 and 1866. They likely came in 1863, the year they took a PEF loan and sailed to America. But the first documentation to place them in Utah is a marriage in September 1866. Further research is needed to narrow the year of their travel.
Birth and death date is confirmed by her death certificate.
Her name is "Ann Bathers" on the 1870 Utah census, "Pheobe Hannah Bethers" on her death certificate and "Phebe Hannah McMillian Bethers" on Find A Grave website.