I was taught that está is used in certain contexts meaning a temporary state (like emotion). Isn't "today" always temporary in that it is constantly changing to the next day? If so, why don't we use está instead of es?
This answer has grown enough to require an outline. Here it is:
1. "¿Qué hora es?" versus "¿Qué hora está?"*
I feel tempted, like others, to write that Spanish uses the verb "ser" to ask the time, simply, because that's the usage, and that one could imagine, like you have, that it would be possible to use the verb "estar". Here, I'm going to propose an explanation based on the usage of verbs "ser" and "estar".
In Sections 2 and 3, I've described the difference in usage between "ser" and "estar". The main idea in that description is that "estar" is used to talk about a state that has changed or that it is expected to change in the future. If you think of "time" as being a clock, as in "What is the time displayed on the clock?", then you could argue that the use of "estar" is more appropriate.
However, when we ask for the time, we are only interested in knowing what the time is. We are not interested in clarifying that we know that the answer is for a specific time. Doing so would feel redundant.
If we ask:
we are simply saying we want to know the time. But if it were correct to ask:
we would be saying not only that we want to know the time, but also that the answer is going to change in the future. Since we don't do these mental acrobatics, that's why we use the verb "ser".
2. "estar" versus "ser"
I reckon "temporary" isn't the best word to describe the usage of verb "estar". I would rephrase this rule of thumb as:
3. "a state" versus "an attribute"
One of the comments to this answer has made realise that distinguishing "state" and "attribute" in the context of this answer is as difficult as distinguising "ser" and "estar".
In the section above, "attribute" is simply used to mean a quality of someone or a property of something. (Note that I'm being careful here and I'm avoiding to say whether this quality or property is permanent or not).
On the other hand, "state" is a quality of someone or a property of something at a specific time. Thus, when we choose to use the verb "estar", we are making the point that we are talking about a state that has changed or that it would be expected to change in the future.
4. "¿Qué hora es?" versus "¿Qué horas son?"
The diccionario panhispánico de dudas explains that, in the educated general register, the question "what time is it?" is asked as "¿Qué hora es?". In this form, "hora" does not mean "hour" but "the time of the day".
This dictionary also explains that the form "¿Qué horas son?", although less recommendable, is also admissible. It is found in some American countries, specially in the popular register.
Another common use of "¿Qué horas son?", cited in this dictionary, is to express disapproval. For example:
The verbs ser and estar, one of the most complex things to explain, since in english estar = to be and ser = to be
SER: to be, as in "To be or not to be", to have an intrinsic quality.
Examples: be a good boy/sé un buen chico. she is a good girl/es una chica buena
ESTAR: to be someplace, to feel some way, to have some relative/external quality.
Examples: she is good looking/está buena. He is busy/está ocupado. He is in Europe. Está en Europa.
Since the time of the day is an intrinsic property of the day, and not some state, place or external/relative property, then you ask
¿Qué hora es?
And please, never, and I mean never, use ¿Qué horas son? or worst yet ¿Qué hora son? unless you want to sound like an uneducated/ignorant person.
The time of the day at any given infinitesimal instant is only one for the exact point where you are standing, so you ask for the time in singular, not in plural, no matter what time of the day it is, 10, 12 or 1.
The only acceptable instance where you can use "que horas son" is when you are generalizing about a certain time period of the day and complaining/asking about people doing certain things:
¿Qué horas son estas de llegar? Literally: What hours are these to arrive?
It's actually more accurate to ask
Regardless, ser goes with time always, because one point in time and space cannot continue, or else we could travel into the future.
There very first rule of