1883.Phineas is born the second son in the Black family. From the start, he never really feels like he fits in with his family--or, if he's honest, like any of them really want him there, though he can't really put that into words, or understand why completely. At any rate, life is off to a less than auspicious start.
1884. A lot happens this year, even if Phineas is too young to understand most of it. Minister Darcy Potter dies early in the year, though Phineas wouldn't know what that's got to do with him. Then the summer brings Plague, fire, and a baby brother, the last of which might not feel like one of the year's disasters if Arcturus got the same treatment Phineas does. It's one thing to feel like your family hates you. It's another to feel like your family only hates you.
Also, the family moves to London, which means little Phineas, since he's a toddler.
1885. In a moment Phineas will no doubt look back on with great fondness later in life, his first act of accidental magic is turning his mother's hair blonde. One day, he's going to think this is very funny. Probably.
1886. Belvina is born. It's kind of novel to have a sister, which is probably the nicest thing Phineas can say about his relationship with any of his siblings.
1889. Cygnus is born, and Phineas' mother nearly dies. Phineas is still young and naive enough to be concerned about this, which is either sweet or very sad, depending on your point of view. At any rate, Ursula doesn't die, thoughtfully giving Phineas many more years to build up resentment towards her.
1894. With Hogwarts looming in the near future, Phineas is, to be perfectly honest, just happy to be getting out of the house. Even if his father is the Headmaster and Sirius will still be at school for another year, Hogwarts is a big place--surely he can carve a space out for himself there. Alternatively, he'll prove that his family is right and that he really is inherently unlovable and a blight on the Black family name. But he'd rather not think about that possibility if it's all the same to everyone else.