"Your birthday's next month"

Question

I believe all of the sentences are grammatically correct (I guess you could argue some are more idiomatic than others, but it's not what the questions asks). Am I right or wrong? If wrong, why?

Choose one grammatically correct answer

• Your birthday's next month, isn't it?
• Your birthday will be next month, won't it?
• Your birthday is going to be next month, isn't it?

I passed this little test. It's from a Russian online recruitment platform. They kind of challenge you to prove you actually have C1 (I don't think it proves anything at all since it's way too short and limited in scope, but whatever). However, there was some single "wrong" answer, they claim (they don't tell me which one). It might be it, I figure

Answer 1

This is probably testing if you know that the simple present can be used to refer to future events, when they happen certainly, due to a calendar or schedule. Birthdays are an example.

My birthday is in September. September is the next month after August. August is this month. Hence my birthday is next month.

You can also say "The bus leaves at 9pm" or "I graduate next year". These events are seen as being inevitable due to the bus timetable, or the University schedule.

So that is probably what this question is about. But all three are syntactically correct. All three express the same general meaning, in different ways. None of the options is clearly incorrect.

"Will" expresses future events, it is a fairly neutral future tense. "Going to" tends to be used for future events that occur as a result of present choices and conditions. "Going to" would be the least appropriate, as the birthday won't occur as a result of the choices we make today. It will happen anyway.

Answer 2

This is ultimately a question of prescriptivism vs descriptivism. In common usage, "going to be" and "will be" are used in effectively identical cases, and "is" can often be heard in many of those same cases as well. But the English grammatical prescription does make a distinction between the three:

• "Is" should be used for the present tense. A birthday happening a month into the future would not qualify.
• "Will" should be used for either facts about the future ("the Sun will rise tomorrow"), promises about the future ("I will help you move house next week"), predictions about the future based on beliefs or opinions, or actions or events in the future that are decided at the current moment.
• "Going to" should be used for actions or events in the future that have either been decided beforehand, are about to happen immediately, or are predicted to occur (with less certainty than a fact) based on existing evidence.

As I said, the differences are extremely minor and quite often not followed in real-world speech, but if the test is looking for formality and pedantry, that's the distinction they're probably using. By those rules, "Your birthday's next month" is probably the most egregious violation, and the one they consider the most incorrect.

It's worth noting that by these same distinctions, the most correct answer would be "your birthday is going to be next month", since it's not a guaranteed fact (in a very sad way) but it's predicted based on existing evidence. However, that's also the option my own experience tells me is the least common in real usage.