What is the best choice when counting days to an event :
18 days left to New Year
18 days left till New Year
18 days left until New Year
The year coming up is usually referred to as the new year.
18 days left until the new year.
18 days left till the new year.
Mean the same, the first is the best choice, the second sounds like a contraction ('til) of the first sentence.
18 days left to the new year.
Would put you a year ahead of where you are today, though people would probably understand what you mean which is :
18 days left in the (current) year.
The informal ways to say this are :
18 days left until the new year
18 days to go until the new year
18 days left to go until the new year