alarm is one of the many commands available in the Tclx extension.
alarm seconds
Instructs the system to send a SIGALRM signal in the speci-
fied number of seconds. This is a floating 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 without the
setitimer system call, seconds is rounded up to an integer
number of seconds.
The alarm command is not available on Windows 95/NT.