if you're forcing the value every frame then it probably won't go off as the game likely checks the time vs alarm when it ticks over internally, which you'd be preventing from happening.
really you want to set it to current system time at startup and leave it.
remember, forcing the system time into memory like this is a hack, it's going to cause abnormal behavior, it isn't how the machine worked, the original machine was 'always on' and once the time was set, would just increment it every 'frame' this cheat is a convenience feature only to work around the fact that with MAME as soon as you quit you turning the machine off, which in reality you never really do.
