No, I don't mean you should always use the 2nd month. Depends on specific conditions, which I don't know about this specific magazine. The way I think about it, there are 2 main cases:
1) magazine is bi-monthly by default. It gets released in one month and covers the next month as well. In this case, I think it's proper to use the first month as a date.
2) magazine got delayed for some reason and had to pretend it covers the previous month, where no magazine was released. In this case, I think the 2nd month should be used, as this is the month that the magazine actually hit the stores.
So, in "normal" cases it's proper to use the first month.
Of course, it might be better to wait for a more "official" answer, I am just throwing ideas around.