He's lived a long time, so long that he has to do the math to figure out how old he is. But numbers are just numbers, in his opinion; marking time in seconds and minutes and hours and days because measuring by quality is just too hard.
It's because of this that he has a hard time attaching years and dates to the memories he's accumulated, except for three.
The first is the day he met Trisha.
She tilted his world on its side. He had believed, until that moment, that he had moved beyond carnal wants and human feelings. He was well on his way to becoming a well-oiled machine, a being without age and possessing unlimited power. And then he met Trisha.
She was exquisite in her country beauty, and he loved her instantly. She was easily charmed—a rather simple girl, for all her kindness, and fascinated with alchemy. She had no talent, alas, but she loved it nonetheless.
He wooed her and married her, and put his plans on hold. That was the beginning of the meaning of time to him, but he was inexperienced.
The second is the day Edward was born.
He had created life before—or some parody of life, perhaps—-Envy, Lust, Gluttony, Wrath. Beings that were his sons and daughters as much, or maybe more, than the crying bundle Pinako placed in his arms. But as he rocked his arms to calm the baby ('it's a boy'), he knew the child would change everything.
He loved the child, but he thought of his other 'children'. He could not wait forever. Wrath would age soon, and grow old, and become useless in the style of humanity.
Then the boy opened golden eyes to stare an infant's stare at him, and he knew the baby was his son more than any homunculus.
The third is Edward's first birthday.
Edward was exhaulting the birthday cake with leaps of joy and childish babbling and frustrated stamping when neither of his parents could understand his chatter when Trisha whispered to him, "I'm pregnant."
He'd stood up a little straighter, and his eyes had widened, and then he had knelt to Edward's furious tugging on his pants leg to lift him into his high chair so he could have his cake. By the time he looked back up, he knew he was smiling, but he felt something hard inside his throat that he couldn't swallow.
In that moment, he saw his life for the next hundred years—-three or four children, grandchildren, great-grandchildren, a dog, perhaps, his loving wife and caring neighbors. It would be full, it would be sweet, and it would be short and insignificant. It would be failure.
He stayed another year anyway, unable to tear himself away from the woman he had married and the children he had begat—long enough to celebrate another birth and another birthday. Edward was showing signs of reading already, chattering to himself over picture books and capturing grashoppers outside to see what kinds of leaves they eat. He adored the second child, Alphonse, and incessantly asked to hold him. His favorite place to sit was Daddy's shoulders, or Mommy's lap. He tried to feed young Alphonse cake on his birthday and was stopped only just in time by a startled Trisha.
He saw his family loved each other, and he looked out the window, towards where Central lay over the horizon.
When he left, he swore to himself he would come back as soon as his task was done—as soon as his goal was achieved. But time, marked in minutes, hours, and days, can fool even him, and his goal is not to be founded so quickly. But he didn't stop counting the days, the months—the years—until Envy came home one day, soaked and annoyed, and announced his fucking wife was dead now, could he please go do something more interesting instead of surveillance?
Hohenhiem grew very still. He almost went back.
Then he recovered himself, and found again the well-oiled machine he had been becoming before his brief tryst.
He looks at the calendar on March 15, and salutes it with a glass of congac. Then he drinks, and closes his eyes, and remembers Trisha, that very first day.