I missed the RRSP contribution deadline (March 2010) for the 2009 tax year. Consequently, my tax owing was much higher than it could have been. Is it possible to contribute to an RRSP and apply that contribution retroactively to a previous tax year, or do I have to claim it on my 2010 taxes?
The answer is no. The "deadline" is called such for a reason. The only case where I have seen receipts issued after the deadline but for the prior year is when the financial institution made a mistake. Even then there is the burden of proof that a mistake was indeed made.
The good news is you don't lose your RRSP contribution room; it carries forward indefinitely. Perhaps in a later year it can again be used to best effect.
Interestingly, RRSP contribution deductions can also be carried forward; i.e. make a contribution this year, but only deduct it in a later year. Sadly, not backwards. The point here, though, is you are not required to deduct your contribution on your 2010 income tax return, either.
Young people just starting out sometimes make contributions but defer deduction until they are in a higher tax bracket later.