Your understanding here is the opposite of the actual meaning.
An until [TIMEPOINT] phrase marks the end of a state or activity. (See this question.)
I worked until 5 o'clock means my work ended at 5 o'clock.
I lived in New York only until I was 17 means that at 17 I moved away from New York.
Adding only to this merely emphasizes that the state or activity does not or cannot extend beyond TIMEPOINT. It is typically employed when the time limit is especially rigorously enforced or is shorter than expected or desired.
You can work only until 5 o'clock; you are not allowed to stay in the building after that.
I lived in New York only until I was 17, when I went away to college; so although I am a "native New Yorker", I know very little about what is happening there now.
Consequently, "You can dance professionally only until you are about 30" means that every dancer's career ends at about 30, when most people pass their physical peak.
Similarly:
- The pup will not live beyond tomorrow if it is not adopted.
- The electrical supply was not increased beyond the point where specific needs were met.
- You should not eat beyond the point where you are no longer hungry.
- Eternal peace will not last beyond the beginning of the next war.
Conceivably, you could use either one.
If you say: These donuts are works of art.
that means each individual donut is a work of art.
However, if you say: The donuts are a work of art.
that means the donuts (collectively, perhaps in box of 12) look like a work of art.
The key question: Is this one work of art? Or twelve works of art?
By the way, the phrase work(s) of art would generally refer to how the donuts look, not to how satisfying they are. If you want to talk about taste and satisfaction, you could use the similar term masterpiece: This donut is a masterpiece!. The word is defined at Dictionary.com as:
masterpiece (n.)
1. a person's greatest piece of work, as in an art.
2. anything done with masterly skill: a masterpiece of improvisation.
3. a consummate example of skill or excellence of any kind:
The chef's cake was a masterpiece.
whereas work of art is defined as:
work of art (n.)
1. a piece of creative work in the arts, especially a painting or sculpture.
2. a product that gives aesthetic pleasure and that can be judged separately from any utilitarian considerations.
That part about "aesthetic pleasure" is a key part of the definition, I think.
Best Answer
Both options are grammatically correct. However, you should probably use a hard day's work instead of a day's hard work.
A hard day's work is a set phrase, even though "hard" is modifying "day." A day's hard work is actually more logical -- "hard work" is a very common phrase. However, native English speakers would not usually say a day's hard work. It sounds awkward, because a hard day's work is much more common -- and to change the order of the words seems like a humorous attempt to misquote the set phrase on purpose.
So I would recommend using hard day's work.
A perfectly good alternative is to say a day of hard work. Or even a long day of back-breaking work.