Till, as a variant of until, is a preposition meaning up to the time of. Till—not ‘til, an unnecessary abbreviation—has been in the language for centuries, and there’s no reason not to use it. To some it may sound less formal than until, but the two words are interchangeable in almost all contexts.
Because many Americans mistakenly view till as incorrect—we’re not sure why this is—the word is much more common outside the U.S. (though until is far more common everywhere). Here are a few examples of the word in action:
It’s less than a month till the World Marmalade Festival. [Guardian]
Kapil Sibal voiced hope that education till higher-secondary level too will become a fundamental right. [Indian Express]
Boeing delays 787 delivery till third quarter [The Australian]
Prejudice against till leads many writers, especially in the U.S., to use ’til—for example:
But other than that, it’s 31 days of counting the hours ’til Daylight Savings Time (March 13) and the Vernal Equinox (March 20). [Seattle Post Intelligencer]
North Texans in for warmer weather, ’til cold front hits next week [Fort Worth Star-Telegram]
In these cases, there’s no reason ’til should not be till.