The reason the person was confused was because you started your question with: "Are you open till...?
"
That is the start of a yes-or-no question, such as:
Are you open till 8 o'clock tonight?
If you are asking for the time, you could have asked instead: How late are you open until tonight?
The
"How late are you..." opening of the question lets the listener know that you will be asking for the time, and not wanting a yes-or-no answer.
ell.stackexchange.com/users/113/J.R.
If you only need to know what time the restaurant will close, you can say What time do you close today?
What time does [name of restaurant] close today?
If you want to know about opening and closing times, you can say What are your opening times today?
What are [name of restaurant ]'s opening times today?
ell.stackexchange.com/users/30978/JavaLatte
What time are you open 'til this evening?
or What time do you close this evening?
(Since it's a restaurant and not a store, I assume they are open late on Christmas Eve.
But you usually need to make reservations, if it's a popular place.)
ell.stackexchange.com/users/42184/Andrew
To ask about a shop or restaurant's opening hours (BrEng), you can use any of these: What time do you close tonight?
What time are you closing today/tonight?
What time is ( the bank , the pizzeria, etc.) closed?
Could you tell me when you're open?
Are you open every day? Is the [pizzeria] open every day?
How late are you open?
ell.stackexchange.com/users/1694/Mari-Lou A
You can just simply say: "What are your hours today?"
If you just want the closing time, along the lines of what you originally asked, you can say: "
Until what time are you open today?"
ell.stackexchange.com/users/46728/Craig
I call a restaurant today and ask them for "Are you open till what time today?" as today is Christmas Eve and the person on phone asked me twice what? What?  So how do I ask for a store/restaurant person on phone about store hours for that day?   You are open till what time?  Till what time is your store/restaurant open today?
How to properly ask for store hours on phone?
ell.stackexchange.com
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ell.stackexchange.com/questions/113367
ell.stackexchange.com/users/30823/javanoob
Possible ways to ask this question include "How late are you open until tonight?"; "What are your opening times today?"; "Could you tell me when you're open?"; How late are you open?"; "What time do you close today?";or "Until what time are you open today?" More understandable alternatives to the original question, whether referring to opening times, closing times, or both, are: " How late are you open until tonight?", "What time do you close today?", "What time does [name of restaurant] close today?", "What are your opening times today?", "What are [name of restaurant ]'s opening times today?", "What time are you open 'til this evening?", "What time do you close this evening?", "What time  are  you  closing  today/tonight?", "What time  is  ( the bank ,  the pizzeria,   etc.) closed?", "Could you tell me when you're open?", "Are you open every day?  Is the [pizzeria] open every day?", "How late are you open?", "You can just simply say:  "What are your hours today?" or "Until what time are you open today?".
More understandable alternatives to the original question, whether referring to opening times, closing times, or both, are: " How late are you open until tonight?", "What time do you close today?", "What time does [name of restaurant] close today?", "What are your opening times today?", "What are [name of restaurant ]'s opening times today?", "What time are you open 'til this evening?", "What time do you close this evening?", "What time  are  you  closing  today/tonight?", "What time  is  ( the bank ,  the pizzeria,   etc.) closed?", "Could you tell me when you're open?", "Are you open every day?  Is the [pizzeria] open every day?", "How late are you open?", "You can just simply say:  "What are your hours today?" or "Until what time are you open today?".