I wrote this before I found your post so it's a little more complicated than you need and would be tricky to adapt for purposes other than being an alarm clock.
If you want to adapt it to do something else when the alarm "goes off", look to this line:
@IF %var_alarm%==%var_time_current% SET var_alarm=Y
Replace the "SET var_alarm=Y" with some other command-prompt command you want executed.
************************************************************************
@ECHO OFF
@SET var_alarm=N
@SET /p var_alarm=Set alarm time in 24-hr time (HH:MM:SS); specify 'N' for NONE:
@SET var_change_bg=N
@SET var_change_fg=N
@IF %var_alarm%==N SET /p var_change_bg=Rotate background color? (Y/N):
@IF %var_alarm%==N SET /p var_change_fg=Rotate foreground color? (Y/N):
@MODE CON COLS=14 LINES=2
@SET var_time_current=%time:~0,8%
@SET var_time_last=%var_time_current%
:TOP
SET var_color_bg=%random%
SET /A var_color_bg=((%var_color_bg%*14)/32767)+1
SET var_color_fg=%random%
SET /A var_color_fg=((%var_color_fg%*14)/32767)+1
GOTO DEC2HEX
:RESUME
@SET var_time_current=%time:~0,8%
@IF NOT %var_time_current%==%var_time_last% CLS
@IF NOT %var_time_current%==%var_time_last% ECHO %var_time_current%
@IF NOT %var_time_current%==%var_time_last% COLOR %var_color_bg%%var_color_fg%
@IF NOT %var_time_current%==%var_time_last% SET var_time_last=%var_time_current%
@IF %var_alarm%==%var_time_current% SET var_alarm=Y
@IF %var_alarm%==Y SET var_change_bg=Y
@IF %var_alarm%==Y SET var_change_fg=Y
Goto TOP
EC2HEX
if %var_color_bg%==10 SET var_color_bg=A
if %var_color_bg%==11 SET var_color_bg=B
if %var_color_bg%==12 SET var_color_bg=C
if %var_color_bg%==13 SET var_color_bg=D
if %var_color_bg%==14 SET var_color_bg=E
if %var_color_bg%==15 SET var_color_bg=F
if %var_change_bg%==N SET var_color_bg=0
if %var_color_fg%==10 SET var_color_fg=A
if %var_color_fg%==11 SET var_color_fg=B
if %var_color_fg%==12 SET var_color_fg=C
if %var_color_fg%==13 SET var_color_fg=D
if %var_color_fg%==14 SET var_color_fg=E
if %var_color_fg%==15 SET var_color_fg=F
if %var_change_fg%==N SET var_color_fg=7
GOTO RESUME