Usage Note: Till and until are generally interchangeable in both writing and speech, though as the first word in a sentence until is usually preferred: Until you get that paper written, don't even think about going to the movies. ·Till is actually the older word, with until having been formed by the addition to it of the prefix un-, meaning up to. In the 18th century the spelling 'till became fashionable, as if till were a shortened form of until. Although 'till is now nonstandard, 'til is sometimes used in this way and is considered acceptable, though it is etymologically incorrect.
The American Heritage® Dictionary of the English Language, Fourth Edition
Copyright © 2000 by Houghton Mifflin Company.
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Sorry to be pedantic, but I just had to add this in here. "Up until now" would therefore logically mean "up up to (till) now", so that would actually make it a redundancy, wouldn't it? So even if lots of people say it, it should, by rights, only be either "up to now" or "until now" (as puffin was undoubtedly taught).