My mother came to the U.S. on a B-2 visa on 01/11/2010 and left on 06/18/2010. On 01/11 she was given a 6-month stay on her I-94. Now she wants to return on 12/24/2010 and stay until 01/25/2011. How likely is it that the Border Officer (1) will not allow her entry in the US on 12/24/2010 based on her prior stay this year, or (2) will allow her to stay in the US for only one month? I know that tourists are allowed to stay in the US at most 6 months in a year, but how should I interpret this rule - is it 6 months in a calendar year (e.g. 01/01/2010 - 12/31/2010) or 6 months in any 365-day period (e.g. 01/24/2010 - 01/24/2011)?

Thank you for your help.