Can I legally write a 2007 check after January 1st 2008?
I've been instructed to do this many times over the years. Is it legal or even acceptable accounting practice?

I am no expert but if it is relating to tax matters, I think it could be determined to be fraudulent. Checks are processed electronically in most case nowadays and if you wrote a check now and dated it in Dec, there would be a long lag between the date on the check and when it was processed. I may be overcautious but I never create a false record. There are so many ways to prove a record is false in this day and age.