Checks are counted from the date they are written and mailed, not the date they are received. So your check will bring matching Challenge funds with it. Thank you.

On a related note, if anyone works at a company that offers matching donations, such as Google or Boeing or Microsoft, your company's matching donation counts with the date your donation was made (regardless of the lag in when your company makes it; donations made previous to the challenge have their company matches not count, and donations made during the challenge do have their company matches count). So it isn't too late to mail checks or do company matches.