(Reuters) - U.S. Treasury Secretary Timothy Geithner is facing growing criticism for his changing predictions on when the country might face a debt default, shifts that have led some Republicans to discount his dire warnings that the debt limit must be raised soon. Since January, Geithner has changed his forecast of when the U.S. would hit its borrowing cap, and the final deadline for raising the debt limit, at least four times -- fueling a belief among rank-and-file Republicans that his latest August 2 deadline is artificial and can be ignored.Some Democrats are increasingly worried that the changing calendar has...