Thank you all for the useful advice!
Avangi, I can understand your shock. Do computer clocks actually go to 24:00 ?? No. Never.
Clive, are you saying that nobody uses zero hour to mean the time period 00:00 to 01:00? I can't remember where I got the impression. Perhaps it was wrong or perhaps I remembered incorrectly, but I somehow thought that zero hour is one of the ways of describing that period without being very exact. No?
Hi, Goodman. A problem with 12:15 am is that people tends to mis-read that as 12:15 pm, especially if they have a preconception the event is going to happen during day time. I occassionally mis-read that myself. I think that could be one possible reason why Avangi didn't notice the coverage was only 1 minute on the date of expiry. If you say something like 00 hour, the readers will at least be forced to stop and think. This may be important if you want to make sure your reader don't misunderstand you. This is, well, just what I think.
EDIT: After re-reading Clive's comment, I think I'd better avoid using zero hour because it would be easily confused with another meaning which is much more common.