half an hour shouldn't make any difference. i was told to be 12 hours apart, and as close to exact as possible, but a few minutes here and there wouldn't matter. was almost an hour late cos i got home later than anticipated from work one night and when i went for my initial BT's to see if hormones were at right level to start the injections, it was all fine, so obviously didn't make a difference. i think if you were regularly letting it go an hour or more over, then you might have an issue, but a small delay shouldn't be an issue

try what i do - an alarm on the phone to remind me every night!!