alarm seconds
              Instructs  the  system  to send a SIGALRM signal in
              the specified number of seconds.  This is a  float-
              ing  point number, so fractions of a section may be
              specified.  If seconds is 0.0, any  previous  alarm
              request  is canceled.  Only one alarm at a time may
              be active; the command returns the number  of  sec-
              onds  left in the previous alarm.  On systems with-
              out the setitimer system call, seconds  is  rounded
              up to an even number of seconds.

              This command is provided by Extended Tcl.