IS A STIPULATION  THAT  SAYS  THAT THE GUARANTY WILL SUBSIST ONLY UNTIL MATURITY  OF  THE  OBLIGATION VALID?

> Generally, no.  Such a stipulation would defeat the purpose of  a  guaranty,  which  is  to  answer  for  the  default  of  the principal debtor.  If the guaranty is only up to the date of maturity, there is no way that the guarantor can be liable since default comes only at maturity date.