�Till', in the sense of �up to a certain point in time' is actually older than �until', both having been around for the past 700 years or so. Shakespeare used it: "Fight till the last gasp" (Henry VI) and Sir Walter Scott used it: "She doubted if the woman would last till morning" (St Ronan's). �Till' is certainly not an abbreviated form of �until'.
Today, �until' may be thought of as marginally more formal than �till' and is certainly more commonly used in writing. However, �till' is probably just as common in speech. It is, therefore, just nonsense to claim - as some do - that �till' is somehow wrong.
The actual abbreviation �til is just a 20th century affectation and of no real significance.