This is likely to be Robert F. Young's "The Dandelion Girl" (1961), as first described in this answer.
It starts out:
The girl on the hill made Mark think of Edna St. Vincent Millay.
Perhaps it was because of the way she was standing there in the
afternoon sun, her dandelion-hued hair dancing in the wind; perhaps it
was because of the way her old-fashioned white dress was swirling
around her long and slender legs. In any event, he got the definite
impression that she had somehow stepped out of the past and into the
present; and that was odd, because as things turned out, it wasn't the
past she had stepped out of, but the future.
The circumstances of his wife's "absence":
When his wife had been unexpectedly summoned for jury duty, he had
been forced to spend alone the two weeks he had saved out of his
summer vacation and he had been leading a lonely existence, fishing
off the pier by day and reading the cool evenings away before the big
fireplace in the raftered living room; and after two days the routine
had caught up to him, and he had taken off into the woods without
purpose or direcion and finally he had come to the hill and had
climbed it and seen the girl.
She's from the future:
"Are you from the city too?" he asked.
"In a way I am," she said. She smiled at him. "I'm from the Cove City
of two hundred and forty years from now."
Her father is a scientist:
"Tell me about your father," he said. "Tell me about yourself too."
And she did, saying that she was twenty-one, that her father was a
retired Government physicist, that they lived in a small apartment on
Two Thousand and Fortieth Street and that she had been keeping house
for him ever since her mother had died four years ago.
Her father has invented his own time machine, though the theory is well-known, and time travel is generally illegal:
"That's because my father invented his own machine, and the time
police don't know about it."
She has one last chance to use the machine after her father's death:
"Will--will you be here tomorrow?"
She looked at him for a long time. A mist, like the aftermath of a
summer shower, made her blue eyes glisten. "Time machines run down,"
she said. "They have parts that need to be replaced--and I don't know
how to replace them. Ours--mine may be good for one more trip, but I'm
The narrator discovers that his wife is the same girl:
Desperate for something--anything at all--to take his mind off Julie,
he went up to the attic to get them. The suitcase fell from a shelf
while he was rummaging through the various boxes piled beside it, and
it sprang open when it struck the floor.
He bent over to pick it up. It was the same suitcase she had brought
with her to the little apartment they had rented after their marriage,
and he remembered how she had always kept it locked and remembered her
telling him laughingly that there were some things a wife had to keep
a secret even from her husband. The lock had rusted over the years,
and the fall had broken it.
He started to close the lid, paused when he saw the protruding hem of
a white dress. The material was vaguely familiar. He had seen material
similar to it not very long ago--material that brought to mind cotton
candy and sea foam and snow.
And the wife's return at the end:
She came forward to meet him, and he saw the familiar fear in her
eyes--a fear poignant now beyond enduring because he understood its
cause. She blurred before his eyes, and he walked toward her blindly.
When he came up to her, his eyes cleared, and he reached out across
the years and touched her rain-wet cheek. She knew it was all right
then, and the fear went away forever, and they walked home hand in
hand in the rain.