The 40hr "full time" work week is gone, and it is because of a 2009 law in the United States that is officially named "The Patient Protection and Affordable Care Act". There is a provision in the bill that states that 30 hours is now considered as being "full time". Anything below 30 hours is now considered as part time.
Here is an excerpt from the IRS.GOV webpage regarding this topic:
"In general, for this purpose, an employer determines its number of full-time employees for a month by counting individuals employed on average for at least 30 hours of service per week during the month or at least 130 hours of service during the month. An employer determines its number of full-time equivalent employees for a month by combining the number of hours of service of all non-full-time employees for the month (but not including more than 120 hours of service per employee), and dividing the total by 120. For example, an employer that employs 40 full-time employees and 20 employees each with 60 hours of service in a month has the equivalent of 50 full-time employees in the month (40 full-time employees plus 10 full-time equivalent employees (20 X 60 = 1200, and 1200/120 =10))."