i met a friend and this was her case. she left for the states for a tourist visa last 2007 she stayed there for 6 mos from april 19, 2007- october 19, 2007. she left america october 19, 2007, . She arrived at her country dated october 21, 2007 because their country was one day advance. When she decided to go back for another vacation last 2008, hawaii USCIS (where her point of entry was)held her for having an overstay for a day or two, as what the immigration officer told her, the last time she left for US year 2007. Her visa was also cancelled.although, they let her enter the mainland for a month, they still held her passport containing her visa. any comments about this because i pity my friend for having suffered that kind of treatment in hawaii.
If she arrived US on Apr 19th, she should have left US by Oct 18th, otherwise it would be considered overstay, but there is no law against those overstay less than 180 days, they may give her a warning.
Even one day too many is overstay , and the immigration authorities have no discretion in the matter.