SQL query for date comparision

Question

Hi everybody...,Gudmorning to all
  This is vishnuvardhan.i have a small dout.That is in my project i have one table that contains username psaaword and last chage date of password.here password is only change by administrator only.y because any error in password formate clint pointout us.Here userid password is use to login clint server.So we give rights to admin only to change all passwords.so here my problem is in admin window one link is ther.that is passwords need to change today.password need to change once in 30days irrespective on working days.total 30 days.I m unable to write SQL query for this.In thta window i have one datagridview.while picking that link the passwords appear which changed before 30 days from today date.Thais is my prob................? Any one Please help me to solve my problem..
Thanks in advance.