Are "till" and "until" perfectly interchangeable at the beginning of a sentence?

Solution 1:

According to Oxford Dictionaries the answer is yes they are interchangeable, but they also offer some style advice about word selection that indicates you might be better off chosing "Until."

till - Less formal way of saying until.

Usage

In most contexts, till and until have the same meaning and are interchangeable. The main difference is that till is generally considered to be more informal than until. Until occurs much more frequently than till in writing. In addition, until tends to be the natural choice at the beginning of a sentence: until very recently, there was still a chance of rescuing the situation. Interestingly, while it is commonly assumed that till is an abbreviated form of until (the spellings ‘till and ’til reflect this), till is in fact the earlier form. Until appears to have been formed by the addition of Old Norse und (‘as far as’) several hundred years after the date of the first records for till.

(emphasis mine)